Colors in visible light result from different light wavelengths. Blue has a wavelength that's shorter than red. There are many different colors of light, and each one has a different wavelength. Eyes are able to detect these different wavelengths because the back part of the eye, the retina, has nerve cells called cones and rods that react to light. The cones react to color. Three types of cones are normally present. These three types react differently to wavelengths that are on the shorter end of the spectrum, in the middle of the spectrum and at the long end of the spectrum.
Monochromacy
In this type of vision, people can't distinguish any colors at all. Because there is no color, the visual world appears as black and white and shades of gray. People who have this condition generally have an aversion to bright light. Also, they can have a condition that causes them to blink rapidly.
Protanopia and Deuteranopia
People with these types of color vision have difficulty distinguishing between red, yellow and green colors. They can, however, distinguish between blue and yellow. People with protanopia can sometimes correctly determine which color is red and which color is green because green appears lighter to them, although not green.
Hereditary tritanopia
People with hereditary tritanopia vision can't distinguish between blue and yellow, although people with this type of color vision can distinguish between red and green.
Anomalous Trichromacy
People with anomalous trichromacy vision can distinguish between colors better than people with protanopia and deuteranopia vision, although the ability to distinguish between red and green is diminished.
