I am bone straight relaxed with Optimum Super and I use permanent color, L'Oreal Excellence Cream, in brown shades about every 2 months or so because I have a lot of gray. I'm about to go back to L'Oreal Natural Match, which was discontinued but I found on Amazon, because it actually coats the strand more making the strand thicker. I'm not after thickness with the dye, but there is a difference in thinness/thickness along my hair shafts where I stopped using the Natural Match and started using the Excellence.
On dye day, after rinsing the dye thoroughly, I DC overnight in a baggy, no heat, with Motions Oil Moisturizer Silk Protein (my DC of choice). I protective style 99% of the time with wet bunning because I avoid heat in general as part of my growth/retention strategy (plus I'm just too lazy to style). I have dyed before relaxing in order to strip some color off and get a lighter brown and even ended up with some blondish highlights on my grays. I suffered no breakage or other ill effects doing that. My last dye/relaxer period, I dyed and then relaxed 4 days later. However, my regimen consists of lots of conditioning (weekly no-heat DCing, daily cowashing, wet bunning with creamy leave in) so my hair is at decreased risk for breakage than someone who heat styles.
I tried the Clairol Beatiful Collection semipermanent color in black and it faded to green on my gray strands...yuck! I'm not a fan of temp colors as the bleed on pillowcases, clothes, etc., and they fade to quickly for someone who washes a lot like me.
I've heard a few horror stories about henna, enough to keep my protein-sensitive head away from it.
L'Oreal brands are generally good to the hair, not as harsh as some Clairol products and better color matching from my experience and reading reviews. People say to avoid the L'Oreal's Feria though which is very harsh and damaging.
The biggest thing I can recommend no matter what you choose is to deep condition right after the dye process!!!
Hope this helps