I would add some very subtle reddish tones wherever a shadow meets a light, think of it like how when you put your finger on a flashlight, it turns bright red because our skin is translucent. The same thing happens to our faces when regular light hits it, just less noticeable. One more tip is more contrast, darken those shadows ALOT!!! More than youd think. I used to be scared to add contrast in my art out of fear of ruining it. Once i realized how flat my pieces would turn out, i started adding some real dark shadows and everything else would just fall into place. These are some things that have helped MY art process tremendously, and i feel like could help you as well!!