Concave sits flusher to skin. So that'd be best.
Loads of people custom make wood, so if you see a wood you think would match (and its skin safe) Choose that!
Does go darker over time with oiling though, so take that into account too.
I think kaos' main problem is that theres a a couple of white shades, and if its off its not tooo horrific (and loads of people seem to have different coloured ears to their head :S). But theres soo much variation in black skin (and asian), they'd struggle to make adequate matches?
It is rather a longer scale. But I've seen them do limited runs before, and its not like they'd need to develop new moulds or anything... Spose if enough people emailed them