Material Enabled Painting
From ZBrush Info
Guess what–not only can you paint depth in ZBrush, but you can paint Materials too. A material in ZBrush is like a material in other programs that provide shaders. It can produce surface effects like wood grain, metalicity, glow, and so on. The difference is that in ZBrush, materials can be applied not just to models, but also to pixols on the canvas itself.
In addition to the number representing depth, this means that ZBrush has another number associated with each pixol, the number of the material applied to that pixol.
Here's a canvas that's been painted with just a couple of strokes. One was done with a reflective material (using a picture for an environment map), another with a 'rough metal' surface, and the final with a 'gel' material. All of the complex surface appearance is due to the material. None of it is due to the painter (me), who couldn't paint such effects if my life depended on it anyway.
These paint strokes were done with depth-painting enabled, so they raised portions of the canvas. Especially with the reflective stroke, you can really see how the material and the depth of the canvas come together to create the final effect.
