Use 1/4 cup chopped, fresh rhubarb and add to 2 cups of boiling water. Cool and strain before using.
Pour 2 cups of boiling water over 1/4 cup of chamomile flowers. Cool and strain before using.
Clarifying citrus rinse
Boil the rinds of 2 oranges and 2 lemons for about 10 minutes. Cool and strain. This removes residue as well, and helps control oily scalps.
Lavender, sage, or cinnamon
Any of these will give you dark highlights. Add 1/4-cup sage or lavender, or 3 broken up cinnamon sticks to 2 cups of boiling water. Cool and strain.
These flowers will give you lovely red highlights. Use dried flowers or herbal tea containing hibiscus. Steep the mixture to a shade you like, then cool and strain before using.