Have you ever tried before? I guess the best way to start this one is by painting on a layer above it with the layer mode set to "multiply" to establish some base colors (preferred light colors) and then on top of it (on another layer) using any brush with the opacity jitter set to "pen pressure" and then lightly start to cover the artwork.
I think i need different brushes. im not very good, BUT... i have started on coloring the drawing pretty good so far, i am using just a round brush, it seems easier to me. I will show you when i finish, so far i have put maybe 5 hours into it.