Chemist John Dalton believed blue fluid in his eyes filtered out red and green, and asked that his eyes be preserved after his death. In 1995, scientists analyzed DNA from the eye, kept since 1844, and found a genetic variation instead. This episode explains color vision deficiency, which affects about 8 percent of men and 0.5 percent of women with red-green types. True grayscale vision, monochromacy, is very rare. Most cases involve the L, M and S cones, either with one missing, called dichromacy, or with one shifted so its signal overlaps another, called anomalous trichromacy, the most common form.

The red and green opsin genes sit on the X chromosome, which explains why men are far more affected, while the rare blue-yellow type comes from chromosome 7. Cataracts, sildenafil and industrial solvents can change color vision too. The episode covers daily friction from ripe bananas to traffic lights, then flips the story: people with deuteranomaly can tell apart shades of khaki others cannot, and reports say colorblind spotters were sought in World War II to see through camouflage. It ends with railway bans, glasses and a cyborg antenna.

• The upside-down Tipperary Hill traffic light in Syracuse, New York, with green above red
• Why a grayscale check is not enough for accessible design, and why shapes, icons and labels work
• Deuteranomalous subjects picked out 15 shades of khaki that looked identical to typical viewers
• After the 1875 Lagerlunda train crash in Sweden, Frithiof Holmgren’s wool matching test led to bans on colorblind rail workers
• Artist Neil Harbisson’s Eyeborg antenna turns color, including infrared and ultraviolet, into sound