For those that don't know, ZBrush is a supplumental tool set (though they have been trying to push to becoming a fully fledged, self-contained kit) for 3D modeling primarily used to create normal maps. ZBrush is somewhat unique in that the model behaves more like a lump of clay than a descrete set of polygons.

Rather than adjusting individual verticies or lines or faces of the model (the standard way to manipulate models in Maya or 3DMax), ZBrush lets you sculpt the model as if it were clay or putty (it then behind the scenes calculates what verticies need to go where to make the model you sculpted). It's a much more fluid mode for modeling for anyone who started their career in physical mediums.

There is one serious problem though, the models created by ZBrush are way too complex for any reasonable machine to be able to handle them. This is why it is primarily used for conceptual work and normal maps (color textures that are calculated based off of higher-polygon models which alter the surfaces of a lower-polygon model to give it a more complex appearance than it actually has).

From my casual understanding, colors in ZBrush are purely ascthetic, since they likely will be completely scrapped when the model is converted into a normal map.