I see two ways to do that. If you have an airbrush and can do those snaky lines, then you would paint the green, paint the brown, and then paint the snakes. Just draw the patterns on and label them G and B, and spray away.
A long time ago, when Scale Modeler was still young, there was an article on painting a Japanese fighter with the green patches over a natural metal aircraft, with the natural metal showing through around the patches. What he did was to paint the aircraft in the green color of the patches, then cut out masks for each of the irregular green patches, and carefully place them on the model. There were a lot of the little green "crooked dots". After all the patches were applied, he burnished down the edges of each one last time, and then shot his Metal paint.
When he peeled it all off, he had what looked like a natural metal aircraft with field sprayed green blotches all over it.
someone else may have better ideas than these two,,,,,,,,,,but, I can say that the method with the bits of tape actually worked for me once.
Rex