Humans normally perceive color within the visible light spectrum of 380-750 nm. Not everyone does, of course. It is safe to say that one in twenty guys (5%) is color blind or more precisely, red-green deficient. This is caused by an X-linked recessive gene. Females can be R-G deficient, too, though much less common - it is hard to receive two copies of the rare gene. An even less prevalent form is blue-yellow deficiency and the most rare is total color blindness. Most eye doctors have never seen the latter two types in their entire professional careers.
Red-green deficiency is just that: the patients cannot readily differentiate, e.g., pink flowers from green leaves, or red from green apples (see images below). They will have trouble with traffic lights if the lights are arranged in an unfamiliar fashion (e.g., horizontal rather than the usual vertical with red at top). Not to mention the inability to tell whether a severe case of sunburn is coming on, a woman is wearing lipsticks, or a piece of steak is too well-done. And forget about reading litmus test results or color coordinating clothing. Otherwise the visual functions are quite normal.
The remedy, although not ideal, is to wear red-tinted contact lens (known as the X-Chrome lens) in one eye only. The patient can actually identify Ishihara color vision test plates just like any normal-sighted person. If you are R-G deficient, a DIY way is to cut out a piece of red mylar file cover, laminate it onto one of the lenses in your spectacles. Put your glasses on, look around, and be surprised at what you have been missing all your life.
(Top: red and green apples; bottom: as seen by a R-G deficient person [from answers.com])There have been attempts at gene therapy repairing color blindness; although it is still unclear if any success (or even necessary).
If a normal-sighted person begins to lose color differentiation and who is afflicted with one or more of the diseases listed below. Then that is not so good because it indicates damages to the optic nerve and/or the retina. And the list is relatively long:
Eye diseases include POAG, AMD, and retinitis pigmentosa. Systemic diseases include Alzheimer's disease, diabetes, liver disease, chronic alcoholism, multiple sclerosis, Parkinson's disease, leukemia, and sickle cell anemia.
Many drugs also can have side-effects that involve disturbance to color vision. Usually your prescribing doctor will have informed you so.
And chemical poisoning by carbon monoxide, carbon disulfide, styrene, and lead can cause disturbance to color vision as well. The most famous example is Vincent van Gogh (1853-1890) who loved to paint in intense yellow (you all recall his "14 Sunflowers in a Vase"). Mr van Gogh was probably a walking medicine cabinet. In his time, he was treated for various ailments which have been theorized to be, take your pick, epilepsy, bipolar disorder, chronic sunstroke, acute intermittent porphyria, lead poisoning, and Ménière's disease. From his paintings, poisoning by absinthe liquor, digitalis, or even lead is indeed quite possible.
