Start in black and white
It’s much easier to paint in black and white at the start because it’s easier to set the highlights and shadows, and shade them. This means you don’t have to struggle with which colours to pick and which to use for the shadows. When it’s time to add colours, you can use Adjustment Layers, and different layer Blending Modes such as Overlay, Soft Light, Multiply and Color. You can add colours on all of these layer Blending modes.