It’s about controlling those mixtures. Certain colours mixed together are going to make mud. It happens easily… here are some of the common reasons that mud happens:
1. Too many colours on the palette – and they are doing their own mixing with very little help from you.
2. The brush didn’t get completely clean, and you are mixing colours without even knowing it
3. The rinse water is – well – muddy. Even just a little.
4. Using a black pigment in mixtures. Since black is already a mix of several colours, it’s a recipe for disaster in most mixtures.
5. You are painting a swamp scene :).
And here’s an important point that flows from #5 – there are definitely times you want muddy colours. They can be beautifully expressive and give a lovely contrast when used near more intense colour. It’s about being able to control when the mud happens – that’s what makes for happy painters and pleasing paintings.