I usually just sketch in my primary fill color already, but if I'm doing something more complicated I also use white color for the sketch. I find it more easier to locate where I need to put everything in afterwards.
Its interesting how alot of people here are painting in what I'd think as "backwards". I always was more a sketch, fill, outline, 3-D, effects, clean-ups last.. kind of guy. Hardly do I ever deviate from that order. Guess its time to break old habits and try something new.