If I'm having trouble with color in a painting it's usually not the color that's the problem, it's the values, the dark, light and halftone relationships. The hues can always vary but the values have to represent what the light is doing in the painting. So a good exercise is to use a palette of white, Ivory black (or an ivory black and ultramarine blue mixture or indigo blue), yellow ochre and Indian red. This limited palette forces you to think in term of values instead of color because you don't have enough color choices to worry about. When we work with a large color palette it becomes harder to find those subtle value changes that define form and depth so a very limited color palette helps you to focus on values, especially those subtle value changes.