Id like some insight on this as well. I used to lighten and bleach all the time. I have had every color in the rainbow lol but I'm pretty sure that most of that hairhas been trimmed off and now I'm dyed jet black and that's all I've been doing for about 2 years now. My last time dying was in January.
@OP I heard that its really dangerous to henna over colored hair though cus it supposedly causes the colored hair to break (YIKES!). I'm gonna search around and see what I can find online in the mean time.