Oh my, thank you for the paint over! This is very helpful.
I knew a few of these things, but I think you represented them better than I would have, so I'm going to try and add a bit more shadows and adjust those lights. The previous bit I uploaded still wasn't finished, and I have since added more, but will definitely take into account the things you've said. You're totally right, those lights need color - not white.
With the rimlight on the backside of her, I was suggesting that the light was being reflected back by something behind her. Not sure if I represented this correctly, or if that's how it would work at all - it has me wondering now.
I didn't intend for the piece to have just a single lightsource either, I was just previously mentioning the single lightsource up there as that was all I was working on at the time. I'm not sure, however, if it would be a stretch for a lightsource to be coming from the viewer, though to have such a dark background with no substance showing either. ^^
I'll add a few of the things you suggested, and get back with the final piece. Anything that I am unable to implement into this piece, I will certainly take into account for the next. Thank you so much for taking the time to help me. ^^