Verdigris
You want your tree to have some character. Make it special. Painting should do one thing. It should put happiness in your heart. You could sit here for weeks with your one hair brush trying to do that - or you could do it with one stroke with an almighty brush. Just let this happen. We just let this flow right out of our minds. Be careful. You can always add more - but you can't take it away. All you have to do is let your imagination go wild.