I ran across this list of herbs and the colors they will produce.

Chamomile:  a lightening herb than can be prepared like tea, cooled then used as a final rinse.  Works best on lighter hair and starts to ahve a noticeable effect after a number of rinses and may take some time to really show.

Rhubarb:  Rhubarb root is the strongest of all herbal lighteners.  Boil the roots for over an hour, cool before using as a rinse.  This is good for golden highlights on light brown or darker hair.  Blonde hair may tend to go more yellow than golden.

Marigold: The yellow petals of marigold and similar flowers have been used for ages to make golden infusion for tinting hair.  

Sage:  A reasonably strong concoction made from the leaves of sage, especially red sage, has darkening properties.  REgular rinsing can tone down greying and highlight dark browns on hair's natural color.  Red sage has sometimes been called the henna of the west, but its effect build up to subtle differences and does not make hair red.

Walnut :  Un-ripened walnut shells can be boiled for about 2 hours to give a permanent brown dye.

Elderberry: Used on black hair  to intensify the color.  Gives a deep shade of purple.

Hollyhock: An infusion of the flowers can be used as a rinse to intensify the "silver" in grey hair.

i also heard that rosemary (like sage) has darkening properties.

All of those herbal rinses are supposed to be safe for hair, right?  Does anybody know the science of why herbs are safe to change your hair color, but artificial methods are damaging?  I know that chemical dyes strip your hair down or something, but what do herb dyes do in order to change the color?

One more question: if you use chamomile or rhubarb, will the lightening effect fade after a couple washes, or does it stay in forever like chemical coloring?

Herbs deposit color on top, so it isn't permanent and doesn't actually change the structure of your hair. That's why they aren't damaging. Think of it like this: mixing some coffee with water and pouring it on your head for a darker color. The coffee can't harm your hair because it just deposits a color on top.