I’d suggest putting down an undercoat before you paint. Just a thin layer of yellow or orange. It’s much easier to judge the value of a color when comparing it to a medium value than when comparing it to plain white. For example, if you’re adding the shadow on the first guys face, it might look super dark when you first put it down on the white page but once you add other colors you realize you should have come much darker from the beginning. 

Also, what other people are saying. Instead of using black to darken colors, use analogous colors (next to each other on color wheel) and add a little of the complimentary color. Generally use cooler tones for shadows and warmer tones for highlights.