Well....I'm glad you asked 
As long as you use BAQ henna (and if you've been to hennaforhair, I'm sure you know what I'm referring to), then there should be no problems with using henna over the top of chemically dyed hair. I am 99.9% sure you won't get a reaction, but I would still get a hairball (from the shower or your brush) and test it on there first.
Now, as for your mix to get a nice brown....you will need to use some indigo with the henna. How much indigo you use will determine the final outcome, but for a medium brown I'd say about a 50/50 mix would do nicely. You can also add other things such as Tumeric (sp?) which will give some nice warm yellow tones. I don't have direct experience with this...but walnut powder is supposed to be a very effective brown dye too, but can cause allergies in some ppl.
If you just want to dye the grey parts of your hair, then I would suggest using full strength mix on those parts, and then glossing your hair with the rest of the mix (ie....mixing it with conditioner or yoghurt to dilute it's dying capacity but will give a nice conditioning treatment to the rest of your hair). If you find there is obvious colour difference between your roots and the rest of your hair, then you could always gloss again with no problems (the same day if you want to!).
Henna and indigo can be obtained through the sellers listed on hennaforhair and Turmeric can be obtained from the supermarket.
Hope that helps you! Please let me know if you need any more info.
(Am posting a new thread about my friend's henna/indigo experience soon....so stick around!) |