A blue rinse is a dilute hair dye used to reduce the yellowed (or translucent, showing scalp colour) appearance of greying hair on older women. The ability to see blue decreases with age, so an older woman perceives her uncolored hair to have a yellow-tinge, and the blue rinse brings the color back to a perceived normal color in their eyes. The blue rinse supposedly makes yellow-white hair appear blue-white, but an inexpertly applied blue rinse will leave a distinctly unnatural tinge behind.
The phrase entered popular culture as a term for elderly women, the blue rinse brigade. An alternative term is "blue hair." It has declined in popularity with the increasing popularity of home dyeing, the reduced prevalence of smoking (which yellows the hair) and with society's more relaxed attitude to ageing.