Thank you for the help, XPTL297. The problem isn't so much the actual darkening of the skins... I've worked in Adobe Photoshop for some 9 years now, so the techniques are fine with me, how to do stuff in the program I know without a doubt.... The problem is that even when I paint a skin from scratch, the translation from my very smooth and pretty template and into the actual ingame look is very poor. That's what I know next to nothing about. It's as if the subtleties of the darker tones just won't show up as nicely as the pale ones do (which sucks, I think).
I never use brightness/contrast because it's a poor tool, and always use levels when I adjust my skins, and following that, often paint over and/or multiply, as well as very carefully change some of the colour values in hopes of fixing the problem. I only use burn and dodge very, very rarely because of the poor control you have over what they do to your colours and much prefer multiply.
It's not even the contrast, for me. The latest skin I'm working on, Pixie Earth, has much less contrast than any of the others and very soft transitions... it's the
transitions that mess with me. I'm going to put it on hold until I can figure out a way to work it, because even a soft noise layer didn't do the trick on this one.
Here's what it looks like before bodyshop mauls it,

On closer inspection, I can't find any dark skins that have good transitions. I guess it's just how the game works, if there's a lot of details, you lose in transitions :/ I suppose I'm just too picky about these things.
Thank you very much for your help

I'm just going to have to keep at it until I get it right.