The Müller Formula (or: Predictable Color Preferences) (Livelygrey)
This site suggests that an amazingly simple rule governs what color combinations most people will perceive as harmonious or pleasing. I’ve been experimenting with it a bit in a paint program and subjectively it seems to hold. To give it away in a few words: in a combination of colors the colors which are darker in value should be closer to purple in hue, and the ones lighter in value should be closer to yellow in hue. Or more precisely, the colors with a darker value should be of a hue which has less natural brightness to it.
(Via Reddit.)