its not too fiddly.
You said your base colour was elf flesh, shaded with ogryn flesh, right?
Assuming the above statement is correct, put some elf flesh on a pallete (or bit of plastic - whatever).
Add a wee bit of water. Not too much - just enough so it flows more smoothly off your brush.
Add a little dark, dark blue or purple. If you dont have such a colour, a mid tone blue or purple would be fine, plus a teeny tiny dab of black.
Mix the above paints together. You should have a colour which basically looks like elf flesh but...unhealthier (and a bit darker. If its not darker enough, muck about a bit by adding more blue or purple, and maybe black. It shouldnt be hugely darker. Say, codex grey is darker than elf grey. That kind of difference. I dunno if that helps...)
paint this into the areas I mentioned before (armpits, knees, underboob, underchin, undernose.) You dont want it right up to the edge. e.g.
step 1 - this was done with 50% transparent airbrush in pshop using dark blue and kinda ogryn flesh colour. Its an approximation.
step 2 - blend in the edges of the shading using the same colour, but watered down more (i simulated this in pshop using a bigger airbrush and 10% transparency). Youll need multiple layers to attain decent opacity. Use this to your advantage to make the transition smooth.
heres a close up showing where I shaded. I based this purely off the shadows in the photo.
give it a shotty - see how it goes. You can always paint it over with the elf flesh again.
PS - you could possibly use ogryn flesh to ease the transition between shadow and base colour, if you were v careful and added just a teensy tiny spot of the blue to the ogryn flesh, but you could just as well just use ogryn flesh at the join between colours and see what happened. tis up to you...
final comparison:
before:after
