What is Color Blindness
Color blindness is lack of color vision so when a person can't distinguish some colors. It does not necessarily mean seeing only black and white (also called monochromacy = total color blindness). Some people can't simply recognize one certain color. For instance, they don't know the difference between red and green. That's why most "colorblind" people can actually see many colors, however, they can't tell a few specific ones.
Our eyes have three color receptors - red, green and blue. That's why our color vision issues are naturally based on those 3 colors and their mixtures. The following pictures depict what the color blind see when they look at colors of rainbow.