if you want to reproduce shading, you'll have to create halftones. this is a pretty significant step up from spot colors - it requires more sophisticated tools and techniques in almost every arena - film, screens, inks and technique (I'm assuming this is a manual job.) You can use photoshop to make halftones, but you'll get better printed results from a RIP. You'll need a typically higher mesh count on your screens (230 tpi) or so, and be able to burn good, hard dots. you need to be familiar with ink additives and what they can do for you, as you'll need to get the ink through the smaller openings in the finer mesh. and, your squeegee pressure and angles need to be consistent if you want your image to look consistent. having said all of this, you can probably bang something out without too much trouble that looks decent. but to get some real quality, some nice tonal variations with good midtone contrast, you'll need to take control of these processes.
