You have a lot of options for coloring but to boil things down there's coloring inside the lines and outside the lines. Outside is sort of like just grabbing a paint brush and slopping some color down, paying no heed to where the lines are...nice for painterly types. Inside is more technical, of course, but you have to do a lot more work to the image before you can get to the coloring part. Mainly, you'll need to remove the grays from the image to leave only crisp black lines. You can do that with threshold, contrast, or lots of hand drawing with black and white. This part is very time-consuming.