Hue, tone and saturation
Hue is the colour name, tone is how light or dark it is, saturation is how intense. Most weak paintings fail on tone, not hue.
Relationships on the wheel
Complementary pairs sit opposite and create maximum contrast. Analogous colours sit next to each other and feel harmonious and calm.
Restraint reads as skill
Three colours plus a neutral is enough for almost any piece. Limiting the palette forces tone and composition to do the work.