Most people are in agreement about the colors of the objects around us. A discussion may arise about whether a flower is violet or purple, but that is usually only a question of the name. Nevertheless some people have a greatly reduced capacity for the differentiation of colors. Such people are casually called ‘colorblind’ although the term ‘color defective’ would be preferable. That people who have poor sight from eye disease often have imperfect color vision had already been noted by Alhazen; in this chapter we shall only be concerned with color abnormalities which are seen in people who in all other respects have normal vision [1].