grayscaling

Awesome
I think that there's grayscaling in Photoshop too if I remember. Good work

Yes the texture is made grayscale in photoshop then colored with geoset animation by the model.
Then you only have to play the color animation you want in-game. Or import some of the standalone models of the color you want. You will get wide variety of colored foliage with only one texture to import.
Some default trees also use grayscaling for extra effects, like magic trees which change color and autumn set which goes from green to yellow/gold/red similarly to snow animations.
Autumn trees can further go into a snow variant.
The snow autumn tree is a bit different than the other snow foliage, the one on top left is the tree.