You can have 'hue jitter' so each stamp of the brush is a different colour. Try selecting the leaf from the Standard brush set. Set foreground and background colours to different shades of green, and in the Brush panel select Colour Dynamics, and set Hue Jitter to 100% and Saturation Jitter to 100%. the Purity slider controls saturation, so keep it between zero and above.
[EDIT] Thrown in some Size jitter for your forrest brush, to give the scene some depth. And mabe a small amount of Angle jitter. Whatever works