Actually you can paint materials directly onto your model. Just make sure that texture: colorize is on then select M only or MRGB and start painting. You must paint with selected material directly with brush to make it permanent, otherwise, every time you switch material the entire tool’s material will change i.e. there is no such thing as material: Fill object like there is for color.
Out of intereset you may want to see a WIP i posted which i think is a good example of painting with materials.
http://www.zbrushcentral.com/zbc/showthread.php?t=48701
Half of the painting done on this space-geared character I made was material painting i.e. the metal, the shiny silver bits etc. It can give nice results but you must becareful while using it like 3dioot said.