Color blindness is a pretty common disorder: about 10% of males and 0.5% of females are color deficient. It is primarily an inherited trait on the X-chromosome, which is why males (XY), with only one X-chromosome, are more commonly affected than females (XX), who have two. Color deficiency can cause a spectrum of disabilities. Most people who are affected have mild symptoms that don’t impact their overall health and happiness. Rarely, people have no color vision at all (Achromatopsia) and this can cause significant vision problems.
There is a nice explanation of what causes color blindness at WebMD:
People usually have three types of cone cells in the eye. Each type senses either red, green, or blue light. You see color when your cone cells sense different amounts of these three basic colors. Most cone cells are found in the macula, which is the central part of the retina.
Inherited color blindness happens when you don’t have one of these types of cone cells or they don’t work right. You may not see one of these three basic colors, or you may see a different shade of that color or a different color. This type of color vision problem doesn’t change over time.
This is an excellent video about what it’s like to be color blind from KidsHealth.org. It is well done and very interesting.
Here is a link to the Ishihara Color Vision online test.