RE: [TECH] Colour Difference Algorithm

Those color combinations are accessibility issues.

A person that is red/green color difficent can not read them.

A person that is red color difficent can not read red against black.  It
all becomes black.

So, yes, they are accessibility issues.

Lee Roberts

-----Original Message-----
From: w3c-wai-gl-request@w3.org [mailto:w3c-wai-gl-request@w3.org] On
Behalf Of Chris Ridpath
Sent: Thursday, September 04, 2003 8:25 AM
To: Wendy A Chisholm
Cc: w3c-wai-gl@w3.org
Subject: Re: [TECH] Colour Difference Algorithm



> "Don't set red on black or black on red.
> Don't set green on red or red on green.
>
A good start. But what are the RGB values, or range of RGB values, for
these colors?

Here are some examples of text using these colors:
http://www.aprompt.ca/redgreen.gif
http://www.aprompt.ca/greenred.gif
http://www.aprompt.ca/redblack.gif
http://www.aprompt.ca/blackred.gif

They might not appeal to your artistic sensibility but are they an
accessibility problem?

Chris

----- Original Message ----- 
From: "Wendy A Chisholm" <wendy@w3.org>
To: "Chris Ridpath" <chris.ridpath@utoronto.ca>
Cc: <w3c-wai-gl@w3.org>
Sent: Friday, August 29, 2003 5:02 PM
Subject: Re: [TECH] Colour Difference Algorithm


> Joe's book and Aries pamphlet are good places to start.  Here's a 
> summary.
>
> In Chapter 9 of "Building Accessible Web Sites," [1] Joe says "If I 
> confuse this item with something else, will I make a mistake? Will I 
> be unable to do what I want?"
>
> "keep in mind you always have to think in groups.  When considering 
> two items, is the first on top of the second? Or right alongside?"
>
> "Don't set red on black or black on red.
> Don't set green on red or red on green.
> Don't place the two halves of a confusable pair next to each other. 
> Don't mix beige/yellow/orange with red and green. UNLESS
> There is no actual chance of confusion.
> The items are widely spaced.
> The items have considerable difference in brightness."
>
> "If, however, you wish to maximally avoid colour confusions, you have 
> a range of colour choices at your disposal. Red/blue
>    Steps: Dark red; medium red; light red; light blue; medium blue;
dark
blue
> Orange/blue
>    Steps: Dark orange; medium orange; light orange; light blue; medium

> blue; dark blue Orange/purple
>    Steps: Dark orange; medium orange; light orange; light purple;
medium
> purple; dark purple
> Yellow/purple
>    Steps (note the restricted list): Yellow; light purple; medium
purple;
> dark purple"
>
> "B-list:
> Brown/blue
>    Steps: Dark brown; medium brown; light brown; light blue; medium 
> blue; dark blue Yellow/blue
>    Steps: Yellow; light blue; medium blue; dark blue"
>
> "You can mix white, black, and grey with confusable colours if the
results,
> given foreground/background combinations, contrast, and other factors,

> are actually unconfusable. You can use confusable colours all you want

> if the confusion has no impact on the meaning or function of the 
> site."
>
> In "Effective Color Contrast" by Aries Arditi, PhD [2], he says 
> "Exaggerate lightness differences between foreground and background
colors,
> and avoid using colors of similar lightness adjacent to one another, 
> eve
if
> they differ in saturation or hue...If you lighten your light colors 
> and darken your dark colors, you will increase the visual 
> accessibility of
your
> design."
>
> (in reference to a color wheel described at [3]),
> "Choose dark colors with hues from the bottom half of the hue circle 
> against light colors from the top half of the circle. Avoid 
> contrasting light colors from the bottom half against dark colors from

> the top half." "Avoid contrasting hues from adjacent parts of the hue 
> circle, especially if the colors do not contrast sharply in 
> lightness."
>
> In other words, Choose dark [blue, violet, purple or red] against 
> light [blue-green, green, yellow, or orange].  Avoid light [blue, 
> violet, purple or red] against dark [blue-green, green, yellow, or 
> orange].  Orange on
red
> (or vice-versa) is not effective because they are next to each other 
> in
the
> color wheel.  Yellow on purple (or vice versa) is effective since they

> are not adjacent.
>
> --w
>
> [1] http://joeclark.org/book/sashay/serialization/Chapter09.html
> [2] http://www.lighthouse.org/color_contrast.htm
> [3] description of color wheel - light colors: blue-green, green, 
> yellow, orange.  dark colors: blue, violet, purple, red
>
> At 03:55 PM 8/28/2003, you wrote:
>
> > > ...However we can take steps to recommend content providers avoid 
> > > using colours which exacerbate the problem. This, in
my
> > > opinion at least, should be where WCAG stops.
> > >
> >I agree. There are certain combinations that everyone agrees are bad.

> >For example the exact same colour used for both text and background. 
> >There
are
> >certain colours that everyone agrees are OK - black on white.
> >
> >But as we move away from these extremes we start to run into 
> >problems.
Which
> >colours do we recommend the content providers avoid? Is it possible 
> >to
draw
> >a clear line?
> >
> >Chris
>
> --
> wendy a chisholm
> world wide web consortium
> web accessibility initiative
> http://www.w3.org/WAI/
> /-- 
>

Received on Thursday, 4 September 2003 09:59:57 UTC