I'm more or less self-taught in Photoshop, which has meant I've had to figure out a lot the short cuts for myself. Up until recently, I've done all my coloring with the brush tool (coloring book style!) but it's always kinda time consuming and since I discovered this easy method for laying down my flat colors, I'm not going back.
I can't remember reading about this particular trick in any of the tutorials I've consulted over the years—so even though I'm sure other people are familiar with it, I decided to put together this mini-tutorial.
The line work does have to be on a separate layer from the coloring for this to work—I colored underneath the lines here as the white space was all transparency, but you can also just set your lines layer to 'multiply'.
I split the tutorial into two images, so click on through:


And just for fun, here's how the finished picture turned out :)