Sometimes, when we paint with watercolors, the pigments clump together in some areas on the paper as the paint dries. This is called granulation.
I suspect (but don’t quote me on it) that this happens more with natural pigments, and less with synthetic ones. For instance, in the painting above, I have used a color called gamboge. Gamboge is traditionally made from resin of the Garcinia tree. I have also used a color called Prussian blue — which is a synthetic color. The gamboge in my sample seem to granulate quite a lot, and consequently — since the green is a mix of the gamboge and Prussian blue — the green granulates, too. But it looks like the Prussian blue does not granulate.
Granulation can create a desirable effect, as it adds structure to the painting. It makes me think of something similar that happens with certain fountain pen inks, called shading, where the ink pools in some segments of a line and as a result creates a more saturated color in those places.