Color handling falls under the first principle of accessibility: perception. The key challenge is accommodating people with color blindness. Before diving into types of color blindness, understand that it's a fascinating yet complex topic. You don't need to master all the details to create web content accessible to people with color vision deficiency. Here's the critical fact: all colors are composed of three primary colors—red, blue, and green. Color blindness types are divided based on this foundation.
Red-Green Deficiencies
The broadest and most common category of color blindness. Not everyone with this type cannot see red or green—they simply struggle to distinguish between them. Not all reds and greens are equally challenging. Some people can easily describe the difference between green and dark red thanks to brightness contrast. Much depends on how similar the red and green hues are. The greater the difference, the easier it is for them to distinguish them. When using images where these colors dominate, account for this. Tools like Photoshop can simulate nearly every type of color blindness—run a test. Is the image still clear?
Evidence suggests that people with red-green color blindness may perceive red and green colors as yellow, orange, or brown. This means yellow, orange, and brown can be confused with greens and reds.
Red Deficiencies (Protanopia)
The color receptors (cones) in their eyes are not sensitive to long wavelengths (red). Reds appear more beige and look slightly darker than they actually are. Greens tend to look similar to reds. While many people with this deficiency can distinguish some shades of red and green, they cannot do so as easily as someone with normal color vision.
Green Deficiencies (Deuteranopia)
In people with this deficiency, the cones in the eye are not sensitive to medium wavelengths (green). The result is similar to red deficiency, except red doesn't appear as dark. They can distinguish between shades of red and green relatively well.
Blue Deficiencies (Tritanopia)
Tritanopia is far less common than the categories mentioned above. Blue deficiencies involve insensitivity to short wavelengths (blue). Distinguishing between blue and green can be confusing, and yellow is also affected—blues may disappear or appear as light shades of red.
Achromatopsia (Complete Color Blindness)
This group represents a tiny minority among people with color blindness. The cones in their eyes are non-functional, so the rods (receptors that distinguish only between light and dark) are their only source of visual information. People with achromatopsia see no color at all. Their world is black, white, and shades of gray. They often have poor visual acuity and sensitivity to bright light. This is the only group for which "color blindness" is a truly accurate label, as all other groups retain some color perception.