How did people find out how to make hair dye? How does hair dye work?

Question: How did people find out how to make hair dye? How does hair dye work?
- Keywords:
  chemical
Asked by aimeeisthebest to Angela, Gopal, Hannah, Becky, Vince on 23 Jun 2013.
- Hannah Little answered on 23 Jun 2013:
  
  People first used plant extracts like henna to dye hair which works by coating the hair with colour. Things like bleach work because of a chemical reaction with the melanin in the hair which can’t be reversed.

Question: How did people find out how to make hair dye? How does hair dye work?