Over time the concepts of colors, and how our brain process the colors have changed a lot, it moved from the belief that objects emit color, to certain objects reflect certain colors, to the modern known fact that objects only reflect their color, for example, a red ball only reflects red wavelength and absorb the rest wavelength of other colors (when illuminated by white light), the brain in the other hand signals the red wavelength and recognize it as red.
So what color blindness is? simply color blinded person cannot distinguish between certain colors, leaving them with a lot of challenges to deal with in their daily life such as recognizing traffic lights, medicine, etc.
In the retina, there are three cells called cones, one for each color, red, green, and blue.
The brain uses signals from cones to determine the color perception, color blindness happen when one or more of the cones are damaged, not working, or detecting different color.
While color blindness usually occurs from birth, or get later in life, yet, there is no cure for color blindness, contact lenses and glasses are available that filters certain color making it easier for the brain to determine the color perception.
-Ali Muhannad (alivwv)