Limit your colour palette

Pick an overall hue for a painting. This can be done with a filter or a brush set to Color mode. Mix a little of that hue in with all the other hues you paint with. This will set the tone for the palette. Use contrasting colour sparingly, saving saturated colour for focal points. Most printers have a gamut range. Routinely convert your image to CMYK to ensure that it stays within that range.