Let's say the memories, ideas, knowledge, information, in one's mind is like a colorful web, a ball of yarn of various colors. Sometimes this may appear as having some form of harmony, and there are whole theories on the harmony of colors just like the harmony of sounds. We should take those "science like" theories with a grain of salt. Aesthetics seem to be too complex, and too subjective, to be described by some simple theory. We do know though that with variable mixtures of blue red and yellow we can make any color, and then by adding black or white we can make a specific color darker or lighter. We can add a specific mixture of both black and white to make a color duller, not as bright. So any possible color that can be identified by the human eye, can be described by ratios of the use of those five elements, really 4 since black and white can be described as one quantity, but practically it is five when you mix colors. There are actually infinite combinations, but there is an estimate of how many can be distinguished by the human eye.