Check out proko's videos on shading, for instance https://www.youtube.com/watch?v=3uEtdDvK6Xo

Hey Matthew, Nice job on the box shapes, they have a lot of dimension. There is a way to make the rounded shapes look more 3D. If you add a core shadow and reflected light, it will really give it a round feeling. I took a couple of your shapes and did a paint over to show you what I mean. Also, when you move through the darker and lighter colors in your rendering of the form, make sure you color shift instead of just adding black or white, this will make your colors look alive. you'll notice on your green shape how I kept adding yellow as I moved more into the light. I hope this helps :)

It really depends on what style you are going for. When I do digital painting I never use air brushes or soft brushes. I paint with a hard brush and mimic how I would paint traditionally. Experiment with different options and find what works best for you. Looking good so far.

These are looking nice so far. What are you trying to achieve? some sort of cell shading? Then I think this is a very good start already. If you don't want the shapes to look absolutely abstract, try painting some cubes or simple structures in the same colour, and find the light and shadow areas within them, they will look more "harmonic" than random shapes :D