straight outta' the box, brushes aren't very effective, but when you use the Photoshop's brush engine and play around with the presets, you can get some pretty good results.

try this:

create a new clean file.

choose two varied shades of green (one for foreground colour and one for background colour).

hit F5 or select the menu option BRUSHES in the WINDOW menu dropdown.

check the boxes next to some of the presets (see attached image).

click ON the preset name to call up the settings (play around with these too...)

see what slight changes to the setting do and find the look/effect you're trying to achieve. 

hope this helps
:)
retrocity