Pure henna (Lawsonia inermis) does not react badly with previously chemically-treated hair. The problem only arises if the brand you buy has additives, synthetic dyes or metallic salts. Read the ingredients or use body art quality henna.
Chemical colour, as well as perms and relaxers, lift the colour to bleach, colour, or reshape the cortex inside. Henna coats the hair on the outside only, glossing over the cuticles. So, if you colour chemically, then henna over it, you will get a reasonably predictable outcome. The henna will varnish the hair tone-on-tone, meaning that if you have highlighted it, the highlights will still show up under the henna.
If you still feel nervous then either snip off a lock of hair, or harvest some hair from your hairbrush to do a test beforehand, and to give you an idea of the colour outcome.