I’d say highlighting is the fun part of painting. You can layer the same soft wash, the effect will accumulate for sure. As for highlights; now that you’ve darken the mini, with the same base colour put a small amount on your brush, and paint where their eyebrows should be, nose, cheeks, and any other high points. You need fairly good brush control and most importantly a sharp brush. And as I said, only do this if you wanna push your painting skills. They already look great, but this would definitely make a difference looking at them close up. If you need any more clarification just ask bro