Buxus

Buxus Dioica (or Katam Powder) is a plant that grows in Yemen and some other countries. 

Buxus dioica is also called Katam or Katam Powder (in Hindi). The leaves of buxus dioica have an ash gray dye. Buxus helps to tone down the brighter color of the henna.  Using Buxus by itself on gray hair will dye the hair a bluish grey color. It is similar to indigo but much milder. You can mix buxus with indigo to get a cooler tint.

If you have tried Indigo and find it too darkening, you can try buxus. Mix it with henna and cassia and you will get ash blonde, milk chocolate brown or medium ash brown. Please note that henna should be mixed with lemon juice and left 8-12 hours to activate the dye molecule before use. Then mix henna with buxus and cassia or amla. You can also mix buxus with indigo to get a more grayish color.

You will get more information about buxus here.
 
 
 
There are many organic ways of keeping (and looking) your hair young and healthy: