It is the interaction of colours within a painting that makes it appear more colourful, not the number of different colours employed. Too many colours can in fact dull a painting - the mixing of pigments leads to dark browns, greys and blacks (as shown by earth and the oil below the surface) and consequently, mixtures employing too many pigments become dull. I aim to use three colours to make most of my paintings (versions of RYB), adding more only if I cannot reach the desired colour combination with three. The limited palette does not mean you cannot use several, or even many, colours in a painting, but in colour mixing a limit on the number of pigments helps protect the mixtures from becoming muddy and dull.