To understand colorblindness, you first must understand how you are able to see colors. Color vision is possible due to the cone photoreceptors in the retina. Each cone is sensitive to either red, green or blue light. If the cones' peak sensitivity is shifted, one will be "colorblind." There are many different types of "colorblindness," more correctly called "color deficiencies." The most common category is a red-green color deficiency. Genetics also play a huge role in color deficiencies, since the genes for the red and green pigments both lie on the X chromosome. Because men have only one X chromosome, they are more likely to be red-green color deficient. The photo below is a great illustration of what the grocery store looks like to someone who is red-green color deficient.