Hi li, THAT LOOKS WONDERFUL AAAAAA!!! Your placement of the colours is so smart, adding shadow at the bottom to blend it to the background and then highlighting the head really pulls your attention there and makes the character look captivating, which is good because your character is very pretty hehe <3 that's actually one of the principles of values and it's amazing that you're already aware of it. And as a pink enthusiast, I'm also in love with the whole vibe of the drawing hahaha
And yes! mid-tones are basically all the colours that would fall in the middle of the tone spectrum in your artwork. So any colour that is not super dark OR super light would be considered as a mid-tone!
And since they're in the middle area, they can be turned both darker and lighter with the overlay filter whereas putting a dark overlay over a dark colour wouldn't really do too much of a difference since that base colour is already dark to begin with! I hope that makes sense c:
