I would start by defining my style - rustic? contemporary? cottage? And then by choosing a set of colors I love, and using the 60-30-10 guideline. 60% of the primary color, then 30 for the secondary color and 10 for accents. So if you like the colors gray, black and red, you would do 60% gray, 30% black and 10% red. Does that make sense? If you have an open floor plan, it's best to keep the colors consistent. If you have medium gray walls in one room you should have gray walls in an attached room, but the color can be reduced by 50% to add interest.