Toward the lower left on the Card and the Chart is a simulation of deuteranopia, a common variety of color blindness.This is from the Greek for "second doesn't see" referring to the second cones in the retina of the eye that respond primarily to green light and are presumed to be defective in this condition.
This doesn't mean that greens are invisible to people with deuteranopia. Human vision is complicated.
The role of the green cones is less to detect green light than to distinguish it from red. Similarly, when the red cones "don't see" (protanopia), the main consequence is that reds, yellows and greens are indistinguishable.
This figure simulates the web-safe
colors as they appear in a deuteranopic condition. In most other
categories of color blindness,
protanopia, protoanomaly and
deuteranomaly, the view is very
similar but there are subtle
differences. In one very rare form, called tritanopia,
affecting thirty people
in a million, colors appear very differently. The Color-deficient vision site
explaining and illustrating these points.
Here is the main web-safe color reference in the Card and the Chart.
In most forms of color blindness, these colors appear similar to those above. When designing a web site, you might try to avoid depending on distinctions that color-blind people cannot make. About 8% of men and 0.4% of women have some form of color blindness.
|Christine Rigden formerly of BT.com has a very informative public service site, Safe Web Colours for Colour Deficient Vision. The chart on the right shows a color scheme with a simulation of how it would look in two different types of color blindness. The colors were chosen to be certain anyone could tell them apart.|
||Thomas Wolfmaier runs the Human-Computer Interaction Resources Network, a repository of wisdom in the quest for humans and computers to get along much better than they do. Some content is under subscription but there's a big searchable collection of books, periodicals and other resources.|
||Vischeck has a terrifically practical service. Forget all the theory, just see what your site looks like to color blind users. It will simulate the appearance of an entire web page in various forms of color blindness. (Select the Deuteranope option for the most common.) It can't convert the colors of certain objects, but it does a great job on most features. Highly recommended.|
The following sites may be helpful if you suspect you have color vision deficiency. They are not meant to be diagnostic. See your vision care professional.
Ishihara Test for Color
Blindness, by Dr. Shinobu Ishihara. This has several online tests.
Color Vision Testing Made Easy,
by Dr. Terrace Waggoner, distributed by Bernell.
This site seems very useful, testing step by step.
Holmgrens Test For Color Blindness This test uses colored wools.