A quick method for separating liquids?
A stable 3-phase liquid system can be created, as Spießbürger mentioned with three mutually immiscible liquids. The best one that I know of is the organic-aqueous-fluorous 3-phase system. 

Fluorocarbons (hydrocarbons in which every hydrogen atom is replaced by a fluorine atom), are interestingly immiscible in both water and common organic solvents. This property has added a dimension to chemical separations using fluorous phase extractions. 

If you have three paints, one oil-based, one water-based, and one fluorous-based, then you could separate them easily using a separatory funnel or similar device. You would have to make sure that the pigments in each paint was only significantly soluble in that solvent, which would take experimentation.