Students often ask me, "What color should I use for the background?"
Background colors can be analogous to the subject in which case there is usually good harmony resulting in a somewhat monotone painting. Neutral colors are also good choices as they will recede bringing the subject closer. Opposite colors are the third choice. Opposite usually refers to temperature (cool vs warm). Careful consideration must be given to dominance. In other words, from a compositional standpoint one would not want equally dominant warm and cool colors. Variety is more interesting.