You may put henna on hair that has been PREVIOUSLY dyed with chemicals. Because most chemically treated dyes contain ammonia, it is very important to wait between 6 and 8 weeks to henna your hair. This is non-negotiable. If you do not wait you could end up with orange hair.
You may also chemically dye your hair AFTER you have henna'd it. You will have to wait between 6 and 8 weeks, to be safe. If you do not wait the ammonia in the chemical dyes can turn your hair green.