Computer colors tutorial

The educational technology and digital learning wiki
Revision as of 22:11, 11 September 2009 by Daniel K. Schneider (talk | contribs)
Jump to navigation Jump to search

This article or section is currently under construction

In principle, someone is working on it and there should be a better version in a not so distant future.
If you want to modify this page, please discuss it with the person working on it (see the "history")

Introduction

Learning goals
  • Understand principles underlying color models used in (simple) computer graphics
  • Be able to specify a color for a web page
Prerequisites
Moving on
Level and target population
  • Beginners
Remarks
  • This is a first draft version. The flash colors tutorial (for now) includes somewhat the same information.

See also: Color, Flash colors tutorial, CSS tutorial and

In various web technologies (such as HTML, CSS or Flash one could distinguish tow kinds of colors, in particular

  • Normal colors, called solid colors
  • All sorts of computer generated bitmap images that simulate special effects, like linear and radial gradients, bevel (3D) effects, shades, etc.

This article concerns (for now) solid colors.

Several solid color models exist, the most popular one is the RGB (read-green-blue) model, followed by the HSL/HSI (Hue-saturation-lightness, hue-saturation-intensity) and HSV/HSB (Hue-saturation-value, Hue-saturation-brightness) models.

Solid colors

Solid colors can be defined in various ways (and there is a whole science behind it). Let's just recall a few principles. For more information, please see the Wikipedia links in the color article.

Let's define a few terms first:

Hue
  • means "color"
Saturation
  • means amount of a color you apply, i.e. the intensity.
Brightness
  • How much light you apply. A lot of light makes a the color washed out and very little light makes it very dark.
Transparency
  • How much you can see trough
  • See alpha channel below
Tint and Shade

According to Wikipedia, “In color theory, a tint is the mixture of a color with white, and a shade is the mixture of a color with black. Mixing with white increases value or lightness, while mixing with black reduces chroma. Mixing with any neutral color, including black and white, reduces chroma or colorfulness. The intensity does not change.” See also the HSL/HSV models.

RGB colors

RGB colors are the most popular ones used in computing applications. A color is defined by the amount of Red - Green - Blue.

RGB is the way computer monitors work. E.g. to get a nice yellow you need 100% Red + 100% Green + 0% Blue. RGB is a so-called additive color mixing model. “Projection of primary color lights on a screen shows secondary colors where two overlap; the combination of all three of red, green, and blue in appropriate intensities makes white.” (Wikipedia). Now if you project each of these primary colors with different intensity, overlapping colors will change.

A representation of additive color mixing (Wikipedia)

This model is not how colors work when you mix real paint. Then you'd rather work with a red-yellow-blue model. Color printers yet work with another model, i.e. magenta, cyan and yellow (or more).

RGB colors can be encoded in various ways. For Internet formats such as HTML, CSS or Flash, most often a hex triplet is used, i.e. a hexadecimal 6 digit number. With 2 hexadecimal digits you can represent numbers in the range of 0 to 255, i.e. the number hex FF corresponds to the normal 255 number. According to HTMLPedia, retrieved 17:38, 11 September 2009 (UTC), “Hexadecimal values specify the three colors of the transmitted spectrum using base 16. In base ten, there are only ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. In base sixteen, there are sixteen digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, A, B, C, D, E, and F. Hexadecimal and octal (base 8) are commonly used in computer programs because it is easy to convert them into binary numbers (base 2). In terms of color, it is common to represent a given color value as consisting of three parts : red, green, and blue, which are mixed together in the specified proportion to produce the final color. A typical color expressed in hexadecimal looks like this: #FFFFFF. [...] A common shorthand version reduces each color to 1 hexadecimal digit instead: #FFF. This allows for 4,096 unique colors.”. Each single digit represents its "double", e.g. #F04 gives #FF0044.

With ordinary numbers you would represent a full red like this:

(255,0,0) - meaning full red, no green, no blue

The corresponding hex triplet is FF 00 00:

#FF0000

In terms of percentage of colors you get:

(100%, 0% , 0%), i.e. 100% red, 0% of green and 0% of blue

Let's now have a look at a few colors in a diagram we copied from Wikipedia on sept 8 2007: It represents "Truecolor", i.e. RGB values in 24 bits per pixel (bpp). In Truecolor, colors can be defined using three integers between 0 and 255, each representing red, green and blue intensities. For example, the following image shows the three "fully saturated" faces of the RGB cube, unfolded into a plane:

yellow
(255,255,0) 		green
(0,255,0) 		cyan
(0,255,255)
red
(255,0,0)		 RGBR.png blue
(0,0,255)
red
(255,0,0) 		magenta
(255,0,255)

For more information about colors see links in the color article. Have a look at Wikipedia's great list of colors if you need to find a number for your favorite color name. (If you speak french, get this one. You also may read the Wikipedia Web colors article. It also includes a list of colors and explains what a hex triplet is.

The HSV/HSB model

The HSV (Hue, Saturation, Value) model also known as HSB (Hue, Saturation, Brightness) defines a color in terms of three components:

  1. Hue, the color: Represented as a position in the 360 degrees of a color circle.
  2. Saturation, the intensity or "purity" of the color: Ranges from 0-100%. 0 means no color, i.e., a shade of grey between black and white. 100 means intense color.
  3. Value or Brightness of the color: Ranges from 0-100%. 0 is always black. Depending on the saturation, 100 may be white or a more or less saturated color.

The Hue scale from 0 to 360 degrees is the following:

The hue scale (Wikipedia)

In many graphics tools (not in Flash) you get a HSV color wheel that looks like this:

The hue scale (Wikipedia)

On the outside you can select a color (H), then on the inside you can select V and S.

For more information about HSV, read Wikipedia'sHSL and HSV article.

The HSL/HSI model

The HSL (Hue-saturation-lightness) model, also known as HSI (hue-saturation-intensity) model is similar to the HSV model described above.

Comparison of the HSL (left) and HSV (right) color models. Source: Wikipedia, retrieved 17:38, 11 September 2009 (UTC)

According to the CSS specification, retrieved 17:38, 11 September 2009 (UTC)

CSS3 adds numerical hue-saturation-lightness (HSL) colors as a complement to numerical RGB colors. It has been observed that RGB colors have the following limitations:

  • RGB is hardware-oriented: it reflects the use of CRTs.
  • RGB is non-intuitive. People can learn how to use RGB, but actually by internalizing how to translate hue, saturation and lightness, or something similar, to RGB.
Advantages of HSL are that it is symmetrical to lightness and darkness (which is not the case with HSV for example), and it is trivial to convert HSL to RGB.

In CSS and the SVG, the HSL value is defined by the position in the color wheel, e.g. red = 0, red = 360, blue=240. Saturation and lightness are represented by percentages from 0 to 100%. Important: "normal" lightness is 50%.

Below is table of the main colors (at 30i intervals). We present three situations:

  • Saturation is set to 100% and lightness to 50% (normal)
  • Saturation of 50% and lightness of 25% (dark)
  • Saturation of 75% and lightness of 75% (pastel)
Hue Color name Sat=100% / Light=50% Sat=50% / Light=25% Sat=75% / light=75%
0 red 0 - red 0 - red 0 - red
30 orange 30 - orange 30 - orange 30 - orange
60 yellow 60 - yellow 60 - yellow 60 - yellow
90 yellow-green 90 - yellow-green 90 - yellow-green 90 - yellow-green
120 green 120 - green 120 - green 120 - green
150 green-cyan 150 - green-cyan 150 - green-cyan 150 - green-cyan
180 cyan 180 - cyan 180 - cyan 180 - cyan
210 cyan-blue 210 - cyan-blue 210 - cyan-blue 210 - cyan-blue
240 blue 240 - blue 240 - blue 240 - blue
270 blue-magenta 270 - blue-magenta 270 - blue-magenta 270 - blue-magenta
300 magenta 300 - magenta 300 - magenta 300 - magenta
330 magenta-red 330 - magenta-red 330 - magenta-red 330 - magenta-red
360 red 360 - red 360 - red 360 - red

Now, where is white an black ?

  • Any hue that is 100% saturated and 100% lightness is white
  • Any hue that is 0% saturated and/or lightness is black

So basically, an HSL color goes

  • From colorful to colorless in terms of saturation
  • From black to normal to white in terms of lightness (50% is normal)

Below is screen capture from the CSS Color Module Level 3 (Working draft, retrieved 17:38, 11 September 2009 (UTC)) that shows on the X axis the saturation (100%, 75%, 50%, 25%, 0%) and on the Y axis the lightness.

HSL Example - 0i red (Source: W3C)

Finally, here are some CSS code examples: 

 { color: hsl(0, 100%, 50%) }   /* red */
 { color: hsl(120, 100%, 50%) } /* green */ 
 { color: hsl(120, 100%, 25%) } /* dark green */ 
 { color: hsl(120, 100%, 75%) } /* light green */ 
 { color: hsl(120, 75%, 75%) }  /* pastel green, and so on */

E.g. The following HSL CSS code 

  <p style="color: hsl(240,75%,75%);"> Kind of not so blue</p>

would show like this (your browser may not support this):

Kind of not so blue

For more information about HSL (and HSV), we point again to Wikipedia's HSL and HSV article.

The alpha channel

In computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial transparency (Wikipedia)

In more simple terms, you can set the alpha to some percentage:

  • 100% can't see through
  • 80% bad see trough
  • 50% in between
  • 30% good see through
  • 10% good see through, but very little color
  • 0% no color left
Alpha color channel

Hint: With the alpha channel you can create other effects than see-through "windows". E.g. you can overlay textures with color or the other way round.

In CSS (if your browser supports it), you may use so-called RGBA values using the %% notation, i.e. The following RGBA CSS code 

  <p style="color: rgba(0,0,255,0.3);"> Kind of blue</p>

would show like this (your browser may not support this):

Kind of blue

Use as background color. The following code 

<div style="background-color: rgba(0,0,255,0.2);">       
<p> Kind of blue</p>
</div>

would show like this (your browser may not support this):

Kind of blue

The CSS 3 specification actually refers to the SVG 1.1. standard, i.e. the chapter on Clipping, Masking and Compositing.

Color names and transformations

In this section will firstly look at color names. Several technical formats define shorter list of predefined color names. Other than that, there exist hundreds of color names whose physical definition differ in various cultural contexts.

We also introduce some transformations, where a program or an author with the help of some graphics software creates "colors" with special effects.

HTML/CSS 2 colors

According to the CSS3 color model, HTML4/CSS 2 includes the following color keywords: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow.

Below is a table:

______ Black = #000000 ______ Green = #008000
______ Silver = #C0C0C0 ______ Lime = #00FF00
______ Gray = #808080 ______ Olive = #808000
______ White = #FFFFFF ______ Yellow = #FFFF00
______ Maroon = #800000 ______ Navy = #000080
______ Red = #FF0000 ______ Blue = #0000FF
______ Purple = #800080 ______ Teal = #008080
______ Fuchsia = #FF00FF ______ Aqua = #00FFFF


SVG 1.1 and CSS 3 color keywords

CSS 3 color keywords refer to the SVG standard that defines many more colors. The model is actually taken from X11 (the graphical user interface display protocol for Unix)

You may have a look at the CSS3 color table that we made from the one in the CSS 3 spec and that relies on the SVG 1.1 specification

Gradients

Color gradients are transitions from one color to another (according to a color space model, typically sRGB, a ). Authors will have to define at least to color bands. E.g. a gradient from white to blue would show all sorts of tinted blues. The most commonly use gradients are either linear or radial

  • Linear gradients: color changing in one direction
  • Radial gradients: color changing from a center to outside

Below is picture we made for the Flash colors tutorial

Linear and radial gradients and Gradient Transform

Colors in 3D graphics

3D graphics languages and tools usually offer a much richer palette of composed color types as well as sophisticated textures that one can lay over the colors.

E.g. in VRML/X3D (a Web standard) you get color types like this:

  • Diffuse color is a color that reflects light depending on the angle of the surface. The object appears brighter (more lit) when its surface is directly exposed to light as you would expect. That's your "normal color".
  • Emissive color defines "glowing objects". E.g. you would use this to build a visible lamp.
  • Specular color defines extra reflection has when the angle from the light is close to the angle you are looking at. It is used together with shininess. You can experiment this effect in real life by holding a (new apple) or a photograph between you and a window (or a lamp).

Improving photo colors

This section will be written later as a separate article. In the meantime, here are a few links:

Use of color in web pages

If you use color in web pages, there are ergonomic and esthetic issues.

Ergonomic issues

Obviously you have to make sure that people can a text, i.e. you should use a combination of foreground and background colors that work on all screens under different lighting conditions. In addition, you may have to create a different stylesheet for printing. E.g. Wikipedia's HTML color names article (retrieved 17:38, 11 September 2009 (UTC)) includes a demo of combinations, some of which clearly don't work.

The ground rules are the following:

  • Use contrasting colors for background and text.
  • Watch out for some "bad" combinations like a pale red on bright green
  • Test the colors on different computers, e.g. As of 2009, most LDS's still can't handle yellow (i.e. it is totally washed out). CCFL back-lit LCD

In addition, you should design a website so that various kinds of color-blind people (1 out of 20 persons) are also able to use it.

Esthetics

As a general ground rule, most web designers believe that one should use a limited set of colors. Some authors refer to "nature", e.g. summer, automn, winter, spring colors or referring to specific kinds of landscapes. The unwanted opposite would be "clown colors".

There are several interesting websites that index good examples. You may study how these use colors

  • Our favorite CSS designs demonstration site is CSS Zen Garden.
  • My favorite tool for creating palettes is Color Scheme Designer, a free online tool that let's you create and visualize palettes very quickly.

Kuler is Adobe's contribution to color palettes sharing. An other nice site that publishes palettes (and sometimes links to enacted web sites is COLORlovers.

Links

General color
  • See also the color article. It includes links to good Wikipedia articles
Specifications
Interesting Browser extensions
  • ColorZilla includes an Eyedropper (you can "steal" a color from a web page), a ColorPicker (i.e. you can select a color from color selection tool), a Page Zoomer and other colorful goodies.
HTML colors

Note: Avoid using any kinds of HTML tags for styling, use CSS to specify colors, the CSS tutorial brievly explains how to do this.

  • CSS Color Values (Note: CSS 2 system default settings seem to have changed in CSS3)
  • Web colors (Wikipedia). A good list of RGB Hex and decimal SVG/CSS3 codes.
RGB color tables with color names
Online tools and web sites for color design
  • Color Blender (Eric and Kathryn Meyer) allows to generate a range of gradient colors. Pick two end colors, then defin how many midpoints you want.
  • Color Scheme Designer. Good online tool for creating and testing color schemes. You first can select the type of scheme (e.g. mono or analogic or contrasted). Then select the range of hues by moving the most important color. Then check for colorblindness.
  • COLORlovers publishes palettes (as well as pure colors and patterns)
  • Kuler (at Adobe). Try out, create and save various color schemes
Other color tutorials
Other kinds of multimedia assets in EduTechWiki

Acknowledgment and copyright modification

Retrieved from "https://edutechwiki.unige.ch/mediawiki/index.php?title=Computer_colors_tutorial&oldid=23358"