I love the effect of granulation, creating visual texture for particular areas of sketches e.g. buildings in shade. The colour I use most that has this quality is Ultramarine Blue. The pigment settles on the paper in a way that isn't completely smooth but textured. This happens particularly with traditional colour like earth colours and cobalts and doesn't usually happen with modern, organic colours.
If you don't want the effect of granulation though, there are many other colours to choose from or you could use distilled water, instead of tap water, to reduce the effects of granulation.