Types of Color-blindness
Color-blindness or color vision deficiency is the inability to distinguish certain shades of color. It is a fascinating topic because of its complexity. There are three types of photoreceptors within the eye. Each is responsible for detecting different color wavelengths. Additionally, there are photoreceptors that detect light intensity, called rods. Color-blindness can occur if any of these types of photoreceptors is missing or functioning incorrectly. The most common type of color-blindness is red-green deficiency—this doesn't mean that red and/or green cannot be seen, but that certain shades of red and green may be difficult to differentiate.
The key principle of web accessibility for users with color-blindness is:
The colors with which they have difficulty distinguishing depend upon their type of color-blindness, but red-green deficiencies are the most common.
Individuals with a red-green deficiency have difficulty distinguishing between some shades of reds and greens, but they can still differentiate between a light color and a dark color. A dark red and a light green will be easy to distinguish because of their contrast difference. People with red-green color-blindness may see reds and greens as yellows, oranges, and beiges. This means that yellows, oranges, and beiges can be confused with greens and reds. Blue is not affected by this type of color blindness.
Protanopia and protanomaly (red deficiencies)
Protanopia and protanomaly occur when the cones in the eye primarily responsible for detecting red are missing or malfunctioning. The greens tend to look similar to the reds. Protanomaly is milder than protanopia, but the result is similar. Many people with protanomaly can distinguish some reds and greens with some difficulty, and, as with protanopia, reds tend to look darker as well.
Titanopia.and tritanomaly are much less common than the other types. Tritanopia is when the cones primarily responsible for detecting blue are missing or malfunctioning. Blues and greens may appear very similar, and yellows may appear as lighter shades of red or may seem to disappear completely.
Rod monochromacy or achromacy (no color)
This group constitutes an extremely small minority among people who are color-blind. All three types of cones or either missing or non-functional, so the rods (the photoreceptors which can only differentiate between light and dark) are the only available source of visual information. Individuals with achromacy see the world as if in grayscale—everything will appear in black, white, and shades of gray. This often results in poor visual acuity and an aversion to bright light.
The term "color-blindness" is most true for this type of deficiency, since these individuals entirely lack the ability to see any color, whereas the color-deficiency is most accurate when people only lack part of the color spectrum.
Designing for Color-blindness
When designing web content it is not necessary to get rid of color. It is not necessary to convert all images to black and white or get rid of images entirely. In fact, it may not be necessary to change anything at all.
Make sure that colors are not the only method of conveying important information.
Most of the time color is irrelevant in understanding content on the web. However, when colors are used to convey or differentiate information—such as a pie chart or bar graph, or using green and red text to indicate pass/fail or good/bad, etc.—this information should also be provided in another way.