From Paul W. McCormack‘s “Mixing Color” article in a previous issue of The Artist’s Magazine:
Adding too many colors to a mixture can lead to mud. As soon as two colors are mixed together, the intensity of the resulting hue begins to diminish. Adding a third color diminishes the intensity even more. If you need more than three tube colors to achieve one particular color note, you may want to rethink both your palette and your plan of attack.