You can use a "scrap" piece of paper/canvas for testing brushes or mixing colours. I'm using the desktop program so not 100% sure how available this might be in the other editions but:

Tools > Reference Options > New Scrap

This gives you an extra little sample to play with. If you mix your colours on there, you can always use the colour picker (or alt+click) to get back to those interim tones as you work.

The colour samples/swatches panel is also really useful. I can't think what sets are built in and which I have picked up along the way, but there's a good oil paint selection based on real life sets. It's a good way to keep things simple and work in a more "real" method than infinite colours.