I'm not sure what would work for you but maybe you can try holding up different colored shirts or something to your face and see what looks best on you. Or if there is a color people say you look good in chances are that would work. If you're worried about the wrong color some dyes stick around a lot longer than others so you could start off with a dye that is known to fade easilly...just in case. Good luck
since henna only adds color, on black it won't show at all, but it will have a red shine in the sun light. (depending on the type/quality of henna it can be from a deep red to a light orange color/shine). On the faded coppery parts, it should make a burgundy color (how dark depends on how dark you hair is now). On your natural color, it looks like it should give a light auburn color (assuming its henna with good/deep pigmentation). Since hena only adds color, its going to look how it does now, only with a red/orange cast to your entire hair.
you didn't mention this but i thought I would share to maybe benefit your or someone else:
If you want a more uniform color throughout, you can mix some indigo with the henna and apply it only to the new growth, this will help darken it to match the darker parts of your hair. You can also try repeated applications of henna to the new growth, this will darken the red. It wont match the brown parts, but it will make it less of a contrast.
My hair is henna/indigoed black and im letting my natural color (dark-medium brown) grow out too. Personally i like the darker ends.
Henna will look great on your hair by the way.
heres my experinece
i did tnr, and them combed them all out after a few weeks cuz i didnt like them and started over natural and i luv luv luv my hair! its been a month and 1/2 and the back of my hair is already sectioned and dreading up. ive got some crazy loops and messy tangles and some of my hair (around my face) is completely untangled and silky smooth still lol. your decision, but im really enjoying the process of watching my hair change and letting it do what it wants to do (with the exception of separating them after washing)
