3 Brush texture
By default, there are four types of brushes in Rebelle. But you can use your own textures. Go to Help> Show Library folder and put your own brush into the Brushes folder (the brush image must be 512x512 pixels and a PNG). A white pixel represents the full amount of colours, grey is half and black pixel is zero.
I created this effect by tapping and using my own texture, with Water set to low.
Rebelle’s Brush engine takes only part of this texture, from the middle of the brush image.