Don't ask me how, since I've not gotten into making brushes yet, but I have seen them like that, but I think they're created in some mystical fashion using "styles' techniques. I had and Ivy brush that gave me the "veining" in dark [whatever coulour you picked] while the main part was lighter. Before anyone asks, no, I don't have it any more, but I do remember getting it at Adobe's Share location. Lou.

"..... and that was when things got interestiing."