The key to mixing clean color is to know your pigments. Inorganic pigments (ultramarine blue, the cadmiums, chromiums, cobalt blue, etc.) tend to make muted mixtures. Organic pigments (quinacridones, phthalocyanines, dioxizines, etc.) tend to make brilliant mixtures.

Try this head to head comparison to understand this issue better:

Take equal parts of Cadmium Red and Cobalt Blue and mix. Expected purple? Nah, you get a brownish purple...a designer might call it eggplant or mauve or something....

Now take equal parts of Quinacridone Red and Phthalo Blue and mix. Voila! A beautiful clean purple that you can tint with white and still keep brilliant.

Does this mean that one type of pigment is better than another? Of course not. Just that if you know what to expect from the pigment, you will save yourself frustration. This will also save you money becuase you will be able to mix the color you want without wasting paint.