I don't really like Zbrush's color square or whatever it is.  So I always use the system pallete button. (Color>SysPallete).  This way I can quickly keep my channels consistent between Zbrush and photoshop.  Works well for making and editing MatID maps.  I've noticed the purer my channels the better results in Substance Painter and Ddo.

I stick to these colors which are "Pure 255-0" and 8 colors are usually plenty for MatIDs.