When painting on a single layer, general rule of thumb is that you block out your midtone colors, add your shadows, and then add your highlights. This way, you don't have to worry about muddying your lightest colors and you have a base to put the darkest tones on and also somethin to blend them with if you choose to. I think the piece could really be improved by painting over some of the places where shadow should be in order to give more contrast and also a sharper edge to things like cast shadows. I can try and do a paint over later.
