CQ, it was fine for me to use PURE henna over my previously tinted hair.
The problems with henna are that there are companies out there (usually over seas ones) who will call their product henna when it has other ingredients that are actually harmful mettalic salt dyes.
You have to be extremely aware of the ingredients in the henna you purchase. If it says 'brown' or another colour, be very careful the other dyes in it are of vegetable origin, like indigo. If the henna comes in a little bottle of liquid I would be very skeptical.
Read the book available for download here: Henna for Hair
PS.. the more applications, the more my curl loosened.