In Brushes, you can always show or hide the navigation bar and toolbar by tapping the screen once.

When painting, tap and hold for half a second to bring up the eyedropper tool. This lets you quickly choose a color that's already in the painting.

While you're painting, double-tap the corners of the screen to access commonly used functionality:

When painting, use the standard pinching gesture to zoom in and out.

During the pinch, you can also scroll by moving both fingers in the same direction. If you lift one finger, you can scroll without changing the zoom level.