Perhaps it's a lousy solution, but I figure one can't see the difference between 0,0,0 and 1,1,1 ..so I stick to 0,0,0 as mask-always. If I need nonmask-black in my images then I use 1,1,1 .. see no evil.
This way I *never* have to think about maskcolors.. it's always 0,0,0 for me.. :P
This way I *never* have to think about maskcolors.. it's always 0,0,0 for me.. :P