FireAlpaca come with a watercolor brush that blend color automatically. I think it is good brush for painting tree leaves. In this timelapse video, I just use the default circle shape watercolor brush to paint the trees.
ArtworkThis is the final artwork which I have upload both in my pixiv and deviantart.
ReferenceDeviantArt Watercolor Brush Tree
Pixiv Watercolor Brush Tree
Youtube Tree Painting