Paint is made up of fixed colored pigments. How can you compute the color of a mixture of different color pigments? Lets say you have a mixture of particles wit

Paint is made up of fixed colored pigments. How can you compute the color of a mixture of different color pigments? Lets say you have a mixture of particles with color c1 and c2 and you know the concentration is x% c1 and (100-x%) c2? The machine at the hardware store can do this but what is the formula?