Red henna
Lawsonia inermis

Instructions for use:
Mix henna with enough lemon juice or apple cider vinegar to make a paste. Leave in a warm place for 12 hours.
Add more of the above to soften the mixture and mix through hair as you would a conventional dye. Wrap hair in cling-film. Leave on for 2 hours. Shampoo out.

100g is needed for shoulder-length hair; 200g for longer hair.