Surface layer mask ?

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Hello, how to make a mask layer for several area?
Because I want to put some color on part of the ground under the trees that will be green and the rest yellow. I use the image map to place the trees.

[Dune]

Why not use a power fractal as a mask for both the green under the trees and the tree distribution?

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I did not do it.  ^ ^

[bla bla 2]

Je ne savais pas le faire, je voulais en gros, mettre deux couleur au même niveau au sol, sur un terrain plat. Ou dans l'image map.

I did not do it, I wanted to basically put two colors at the same level ground, on flat terrain. Or in the image map.

(google translate).