screen and overlay. One knocks out the white, the other the black. Slips my mind at this time, and i don't feel like checking. I use that method to color drawings, keeps your lines always on top, and the color neatly below. Also, double click the layer (i think) whatever it is you do to get to the layer options. I've been using PS for at least 8 years, so i just do things by instict, so I honestly couldn't tell you what keys, i just press away, a real power user. But you can adjust how the layers blend and such, try every options so you know the limitations of each. It might be a good idea to convert this black and white drawing to actual black and white pixels first then back to RGB. I would adjust the contrast slightly to make sure white is white and your lines are true black for the best results.