Understand the canvas
A canvas texture is made up of a seamless black and white image maps, but you can use any .PNG file, even if it’s not seamless. How colour interacts with the canvas isn’t only governed with the brush’s Pressure setting. At the bottom of Brush creator dialog there’s a slider that controls how much of the Canvas’s texture is on show. This enables you to use the texture as a mask of sorts, as you control how much colour appears on the canvas.