Overall, most boxes of permanent commercial color advise you not to use the product over henna. I would wait 6-9 months or more after your last henna treatment before I used anything besides semi permanent color, such as a rinse.
If you have been using unadulterated, BAQ henna, yeah, you can dye over it - definitely do a strand test first.
If you are the least bit doubtful about the source/ingredients in your henna - I would wait a month or so, then strand test......
Overall, most boxes of permanent commercial color advise you not to use the product over henna. I would wait 6-9 months or more after your last henna treatment before I used anything besides semi permanent color, such as a rinse.