Most people see color through three cone types tuned to short, medium and long wavelengths, but the common ancestor of all vertebrates had four. During the nocturnal bottleneck, when early mammals hid from dinosaurs, rods mattered more than cones and mammals lost two cone types, and primates later re-evolved a third, mainly to spot ripe fruit. Birds kept the fourth cone and see into the 300 to 400 nanometer ultraviolet range, so zebra finches that look alike to us carry distinct UV plumage patterns, and pigmented oil droplets in their photoreceptors sharpen each color channel. Daytime raptors shifted that cone toward violet, trading UV for sharp focus because short wavelengths cause chromatic aberration.

Humans can rebuild the fourth cone through the X chromosome, where the red and green opsin genes sit side by side. A woman who inherits a variant opsin from a father with mild color blindness can, through random X inactivation, develop a mosaic retina with four photopigments, and estimates of such carriers run from 15 to 50 percent of women. Most remain functionally trichromatic because the brain lacks a fourth opponent channel. Neuroscientist Gabriele Jordan searched for 20 years before subject cDa29, in 2010, reliably told pure orange light from a red green mixture in a Rayleigh match.

• Mice engineered with a human cone pigment that gained measurable color discrimination
• Conditional tetrachromacy in twilight, when rods peaking near 500 nanometers feed into color processing
• Aphakia, where people whose lenses were removed in cataract surgery report seeing ultraviolet as a ghostly whitish blue or violet
• Pentachromacy in some lampreys and pigeons and 15 opsin classes in the bluebottle butterfly
• The mantis shrimp and its 33 distinct light detecting proteins