http://www.hennaforhair.com/faq/index.html
Some information here HTH
Warnings!
What will henna do to your hair?
Compound henna does NOT play nice with commercial hair dye and other chemical residue in hair.
People who have itching, blisters, or serious allergic reactions to para-phenylenediamine hair dyes often want to change over to henna hair dyes.
Compound henna often reacts badly with synthetic hair dyes and other chemical hair products! If you use compound henna soon after you have done your hair with para-phenylenediamine dyes, your hair may be brittle, trashed, and may turn frog butt green or offshore sewage outlet black. Compound hennas have metallic salts which react disasterously with the ammonia in synthetic hair dye. Problem is ... many pre-mixed henna powders do NOT have these ingredients listed!
Want to learn more about compound henna? Go here,.
http://www.hennaforhair.com/warnings/index.html
Body art quality henna can be used over commercial hair dye and other chemical residue in hair.
Body art quality henna does not have metallic salts, and can be applied over synthetic dyes! How do you know if you've got body art quality henna? Know your supplier! Mehandi, and the other suppliers listed at Henna for Hair Suppliers have body art quality henna!
The only way to find out if YOUR henna will cooperate with YOUR chemical residue from synthetic hair products is to TRY SOME. Harvest hair from your hairbrush, mix up a sample of henna and try it. What's in some henna products that reacts badly with synthetic hair dye? Here's the low down and dirty about what's in some of these products!
Do a spot test to be sure you're not allergic to henna. Mix 100% pure body art quality henna with warm water. Wait an hour. Put a spot of henna on your inner arm. Wait an hour. If the spot doesn't itch after an hour, you are not allergic to henna.