How to make full color brush in photoshop?
Photoshop brushes do not contain color information. All color is derived from the foreground/background color settings and any brush dynamics, such as the Hue Shift setting.
Brushes are greyscale images in all instances. There is no way to alter that.
You may want to look into smart objects and duplicating those to create multiple instances of colored artwork. This would be the most efficient method. It would allow you to use the same tree over and over but you couldn't simply "brush" the artwork on.
Photoshop CC also has the Tree Filter located under
Filter > Render > Tree. . . which is designed to create trees of various appearance. Exploring that may be of benefit depending upon what you are trying to achieve.
It is possible to get Photoshop to create multiple trees along a path using the