Color blindness is of many types (total and partial/ inherited and acquired/ red-green, blue-yellow types, etc. ). But usually when we talk about color blindness, we consider the most common form, which is the “red-green” type.
Color blindness is a chromosomal abnormality. It occurs because of the presence of recessive genes on the X chromosome (so the name X linked inheritance).
There are three genes that are responsible for production of the photo pigments by the M, L, S cones in the retina of the eyes.
OPN1LW gene helps in production of photo pigments for L cones that are sensitive to red color.
Similarly, OPN1MW is for production of photo pigments for M cones that are sensitive to yellow-greens and OPN1SW for S cones sensitive to blue color.
Mutation in these genes causes damage or defect in the production of specific photo pigments that leads to reduced or completely lost color receptivity.
Note that the X chromosome is one of the two sex chromosomes (the other one being Y). Now since this is a recessive gene, it will get expressed either when it occurs singly as in males (they have only one X chromosome). For it to get expressed in females, it should occur in both their X chromosomes, which is slightly rare but not impossible. In females, even one chromosome with the normal gene does the job perfectly.
The symptoms of Color blindness are partial or complete inability to distinguish between colors. For instance, in the red-green types, the person cannot distinguish between the shades of red and green colors. Though it rarely happens, as commonly thought that the person sees the world as just grey, usually he/she can’t simply differentiate between reds and greens, blues and yellows, etc. The person cannot be accurately called blind, but rather color vision deficient. The whole eye is functional except for the retinal cone cells that are either faulty or totally absent.
The most common test used to detect color blindness is the Ishihara’s test. A color blind person usually fails to read the numbers mixed between the colored dots on a series of cards. A Color Arrangement test, which makes use of a color palette, can also be used.
There is no cure of color blindness as of now. One can use corrective tinted lenses, but these may or may not act as a complete solution.
The gene mapping that you may be talking about to repair the damaged genes had shown some success in color blind monkeys but it is not considered safe for humans. Looks like there isn’t much scope as of now. Most people have to learn to adapt themselves to live with it. Besides, this can be a benefit sometimes as a color blind person can easily manage camouflages.