Say, from grey to green?
Nope. The mask is always grey. Or to be more exact, it’s a darkening of whatever color the object already is.
matt is correct but you can color with a mask…just mask a area you wish to color and click the texture/colorize button and your off coloring you tool…
Did you just call me a tool?
lol I think he meant your tool, not you tool
Evoken,
That is a very good point you have raised. It should be on the next ZBrush wish list update.
In the same way that masks can be coloured in other masking programs like photoshop. So if you are working on a red object you can have a cyan mask.
It would be especially useful where the mask is painted with a soft brush and it is difficult to see the transition with grey.
Or you can choose whatever colour makes the mask easier to see.
You tool??? bwahahahahahahahaha…ROFLMAO
I just read through that and busted a gut… 
You are all craaaazy! My stomach hurts now, thank you very much.
lol and no i didnt call you a tool… typooooooooooooo…lol (YOUR TOOL)
He said “TOOL” ! hehe hehe