I would agree with Seamonkey
i agee with drobbins who agrees with seamonkey, no really, what kind of project, what kind of drawing, what is the subject matter, there is more than one way to add depth to a piece other than color, perspective also creates great depth, even with out color. aerial perspective, linear perspective, vanishing points, cool color, warms color, lots of differnt way of doing it, depending on your subject, the materials, the colors, or are you talking bout adding depth with a nice thick clear coat.