This exercise is very good for everybody who used to paint with ready triads. If you want to try painting without limiting yourself to ready colors straight from the pots, I recommend such an exercise before moving on to the miniature:
Choose the colors that you want to use for painting the model. Start with main, dominating colors, and finish with minor, marginal colors. Then think which colors will be placed next to each other and mix them in various proportions to see which mixes could be used for shading.