If you need a brain break and can't figure out what color to color what part, then try going monochrome with a simple gel pen (make sure it flows smooth or it'll drive you crazy).

colored this whole sheet in a dark blue ink and I loved the way it turned out in the end.

If you want some added shading, just find a pen that is slightly darker or lighter than your main color. You may not be able to tell the contrast at first you will see it at the end.