Here some explanations and the script. Always use it with a small amount of vertices on the mesh. When OK augment the number of vertices.
The first idea is to use Noise for the coloring. There is first a threshold to make Island of color. If the value is -1 there are just one part
To make the white (grey here) the trick is to have white at the lower bound of the gradiant
The integer adapt the general scale of the noise
The B adpat finely the scale of the noise
The second B gives some more noise at a lower scale
erindale watercolour V2.gh (30.3 KB)
I must then try to use it with GHShader nodes …