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Color Blindness

From: <nir.dagan@econ.upf.es>
Date: Thu, 20 Aug 1998 12:35:34 +0200
Message-Id: <H0000e2200af1bc0@MHS>
TO: hbingham@ACM.org
CC: greglo@microsoft.com, w3c-wai-gl@w3.org
In the era of stylesheets color blind people may override all 
author defined colors, asuming their browsers 
are designed for usability. To a large extent this would be 
a solution.

There are web creatures in which you cannot override colors, e.g.,
images of types used nowadays. (vector graphics may be different)

Also some older browsers do not allow to override colors defined 
via table cells (TD) or the FONT element. And some recognize 
the FONT color, but not the TD background color.

So my recommendation is first to promote the usage of stylesheets 
for colors (without restrictions concerning color blidness) and 
put all the color blind restrictions with respect to images.

Using FONT and TD for colors should be generally discouraged 
as it gives trouble to everyone.

Nir Dagan.

>Thanks to Greg Lowney for important info on color blindness in 
>"But Can They Read It." 
>That material explains a lot, but doesn't repeat specific
>color choice contrasts and reasons as in your primary reference.
>    http://www.lighthouse.org/1lh32a.html
>That reference gives examples, then explains the three dimensions
>available for choosing colors: lightness, hue, and saturation 
>in a three-dimensional color space.
>[My paraphrase: Lightness is grey scale, white to black top to bottom.
>Hue is spectrum of colors of rainbow, joined onto itself around
>a circle at red-violet. Saturation is color intensity: grey at center to only 
>pure color radially at the outside.]
>The three recommendations there are illustrated:
>1. Exaggerate lightness differences between foreground and background
>     colors, and avoid using colors of similar lightness adjacent 
>     to one another, even if they differ in saturation or hue. 
>2. Choose dark colors with hues from the bottom half of the hue 
>     circle in Figure 7 against light colors from the top half of 
>     the circle (or white). Avoid contrasting light colors from 
>     the bottom half against dark colors from the top half (or black). 
>3. Avoid contrasting hues from adjacent parts of the hue circle, 
>     especially if the colors do not contrast sharply in lightness. 
>I found another good reference:
>Web Page Design for Colour Blind Readers
>    http://www.cimmerii.demon.co.uk/colourblind/
>Included there are a different list of DO's and DONT's that would
>be useful for WAI guidelines, with specific color problems
>DO provide ALT="..." text for all your images. If a user cannot 
>understand your image they can reload with images off.
>Consider using JavaScript MouseOver events to provide status-line 
>descriptions of images- especially maps and navigation bars. 
>DON'T use [red | green | brown | grey | purple] 
>           [next to | on top of | changing to] 
>          [red | green | brown | grey | purple]. 
>DO have a strong, bright contrast between foreground and background 
>colours- not only for your page text but also in your images. 
>Even totally colour blind readers can differentiate similar colours 
>which contrast bright with dark. 
>DON'T use colours in images to denote special areas, such as bar 
>charts, maps and navigation bars. Consider using textures or line 
>shading instead (try the "paper" or "pattern" function in your 
>graph or painting program). Alternatively, provide 
>additional written labels. 
>DO use blue, yellow, white and black if you really must use 
>colours to distinguish items. These combinations are less
>likely to be confused than others. 
>Regards/Harvey Bingham
Received on Thursday, 20 August 1998 06:27:19 UTC

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