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In response to this question about iconography, I was wondering about how much it is necessary to factor in color usability for people with color perception difficulties into a design.

The first instinct is to say of course it's very important, but note that I don't mean the question as 'How much do we have to bother about colour?'.

What I'm wondering is whether people with color blindness are so used to dealing with colors around them in their everyday lives, that they barely even have to think about interpreting the colors - provided there are other indicators to assess the differences.

For example - a typical scale or range of colours is red/amber/green - used internationally for traffic lights, progress indicators, status lights etc - in software and hardware.

Is it necessary to re-assign these colors so as to cater for every type of color blindness - (is it even possible), or are people with color blindness quite used to dealing with (or working around) these colors, making it unnecessary to actually change the colors away from these standards.

So - in addition to secondary (or even tertiary) indicators such as symbol, shape, size, position, number and labels, how important is it to consider the actual color for color blindness in a design.

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I know that there is software and monitors that correct colors for color blind people. Does anyone know what's the percentage of color-blind people who use them and how much does it help? – Dmitry Semenov Aug 1 '11 at 9:56
Just wanted to give a link to a project developed in Portugal. It's called ColorADD and it aims to be a standard on how to present color to color blind people. It's being use in coloured pencils (in portuguese), subways and hospitals, for example). Anyway it may be useful for someone. I'm not affiliated with the project in any way. – jpsstavares Jun 18 '12 at 9:18
Red / Green colour blindness is surprisingly common: eg "In the United States, about 7 percent of the male population – or about 10.5 million men"… – PhillipW Jun 18 '12 at 12:44
up vote 14 down vote accepted

Contrast ratio is far more important than the colours you use. The W3C are more concerned with contrast for achieving a AA rating for Accessibility. That's not only good practice for colour blindness but also for general usability.

A tool like the Colour Contrast Checker from (screenshot below) is a handy quick reference tool to check such ratios:

enter image description here

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+1 for a great tool – Steven Noto Aug 1 '11 at 16:59

Color is not anywhere near as important as contrast, as Jon W has also stated. There is really no way to consider colors for the color blind because as you had on you post:

enter image description here

both the green and red clocks look almost indistinguishable in the lower portion of the image, especially if you were to use a lighter shade of red.

And this is only considering one of the forms of color blindness...

So - color should not have much relevance unless it is a color that is expected in a particular position, like in a stop light. In cases like these, although color is not the major contributor to comprehension, if there is a standard it should be adhered to, as to not add anymore confusion.

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Unless color is crucial to a branded experience, I usually try and avoid having color denote anything.

Why not write a clear task message, and make the corresponding control support that?

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I think that the emphasis in the question and its title should be "in addition to ... indicators such as symbol, shape, size, ..." (and not just non-emphasized sentence near the end).

It is very important to take color blindness in consideration, however, if after converting your site to greyscale you can still use it, then your site is usable.

For this, you need to make sure your site uses contrast and styling to differ between stuff and not only color.

Also, different types of colorblindness cause different effects, therefore, you can not choose sets of colors that suits everyone (except for greyscale) - only contrast levels and shapes.

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