These may be caused by illness or injury. Color discrimination also declines with age. Red-green defects occur in trichromatic and dichromatic forms. For blue-yellow defects, only a dichromatic form has been generally accepted. Individuals with red-green defects confuse colors from red through yellow to green.
Protan defects have a trichromatic form, protanomaly , and a dichromatic form, protanopia .
Deutan defects also have a trichromatic form, deuteranomaly , and a dichromatic form, deuteranopia . Individuals with blue-yellow defects confuse colors from yellow through green to blue. There is only a dichromatic form, tritanopia . The existence of a trichromatic form, tritanomaly , has not been confirmed. Trichromatic forms are thought to be caused by dysfunctional cones.