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Re: Colors and the WCAG

From: Joe Clark <joeclark@joeclark.org>
Date: Sat, 19 Oct 2002 18:05:22 -0400
Message-Id: <a05111a06b9d780f6aa81@[]>
To: Charles McCathieNevile <charles@w3.org>, WAI-IG <w3c-wai-ig@w3.org>

>>..which is virtually impossible for a layperson to estimate. 
>>Quickie question to the WAI: What exactly do other people see?
>Not speaking for WAI, just myself, the answer is I don't know.

The answer for all human beings throughout history and into the 
foreseeable future is "I don't know." It is physiologically 
unknowable. Even-- actually, *especially*-- leading scientists in 
colour vision are extremely reluctant to speculate about what 
colour-deficient people see because such researchers can, at best, 
rely on inferences from empirical testing and from interviewing such 
people. (The most valuable subjects are the eight people documented 
in the literature as having one colour-normal and one 
colour-deficient eye. Those people can compare and contrast just by 

The WAI must exercise much more caution on this question. WCAG 
readers should not be encouraged to jump to the conclusion that 
certain colour combinations are guaranteed to be safe, a claim it is 
empirically impossible to prove.

>  I believe (reaching into my memory, which tells me someone posted a 
>proper reference to this list once which I think is the basis for 
>what I am saying) that having something like a 50% or greater 
>difference in values of saturation and brightness, combined with a 
>90-degree difference in hue, provides enough contrast that most 
>sighted people can usefully distinguish things.

So let's see.

We now expect Web designers-- ignored, derided, and misunderstood for 
more than half a decade by the WAI-- to begin engaging in 
calculations of colour values, using two different units, on every 
page of every site they design from now on.

Can you tell me where in Photoshop or Fireworks I can set those 
things up? Just real quick?

>  Of course there are some people who use black and white only,

I'd like to know where they're getting their monitors. Ever tried 
buying a greyscale monitor these days? (Not from an antique store, I 
mean.) Unless of course you mean Lynx or equivalent, which can and do 
run in full colour. (I could give you a screenshot of my own usage.)

>  and large size (at least there are some friends of mine who do 
>this. I suspect they are not the only ones.)

"Large size" and "colour" have little correlation.

In practice, even confusable colours (a spectrum that extends beyond 
red and green) can be used together if other factors make confusion 
impossible or, since we cannot definitively establish what other 
people see, manifestly unlikely. Read Chapter 9 of my book for 
everything you ever wanted to know, backed up by actual research 
citations and extensive interviews with scientists studying colour 
vision. That level of research should have been the minimum used by 
the WAI in establishing its guidelines for accommodating 

     Joe Clark | joeclark@joeclark.org
     Accessibility <http://joeclark.org/access/>
     Author, _Building Accessible Websites_
Received on Saturday, 19 October 2002 18:06:18 UTC

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