The human eye contains rods which detects levels of light (dark versus bright) and three kinds of cones which distinguish among the different colors. If none of the cones are functioning (very rare), then a person has grayscale vision. If the cones are semi-functional (also rare), then the person sees colors, but they are muted.
More commonly, a person with color deficient vision has a difference in only one of the cones, and it is usually either the red cone or the green cone. The result is that some colors appear to be more similar or are muted. The different type of deficiencies are simulated below.
Some color blind users are lacking the capability to detect the lower color wave frequencies associated with red. For these users, red color waves read as "no signal", or "black". These users confuse red and black, so this contrast should be avoided whenever possible. Red and white is legible, but indistinguishable from black and white.
Sample Warning Signs
Note that the black text on red sign becomes black on black for some color blind users.
|Red Vision Missing||Warning||Warning|
If one or two sets of cones do not function correctly, then a person will have trouble telling certain colors apart. Almost 10% of men are red/green color blind; another group are blue/yellow color blind. Despite the fact that red-green contrasts are very distinct for about 95% of humanity, there are about 5% of people for whom this is non-functional. This is exacerbated by the fact that red and green are nearly identical on a gray scale monitor. See examples below.
NOTE: Red-Green color blind people are better able to find camouflaged objects in natural settings. See Visicheck Article for more details.
The paragraph above is "Merry Christmas" in green text on a red background which a color blind person may not be able to read. This is also problematic because it causes a visual vibration for users with normal color vision.
In grayscale, there is only a slight contrast because red and green are of nearly equal brightness.
In Deuterope Mode (Red/Green Color Blindness)
Appears as dark olive on medium olive in normal color vision. Simulation courtesy of Visicheck.
In Tritanope Mode (Blue/Yellow Color Blindness)
Actually appears as teal and magenta. Simulation courtesy of Visicheck.
More Accessible "Merry Christmas" Banner with Yellow Background
The yellow Background separates the areas of red and green and provides a contrast in brightness, making the sign more legible. See the grayscale example below.
See the list on the introductory page about color deficient vision.