I advise you practice first...I use a specific technique to colour my pictures, you should try developing one yourself. Makes life that bit more easier when you want to CG something.
TOP TIP: Instead of colouring onto the image directly, get rid of ALL THE WHITE in the background and base of your image so you just have the outline. Copy and paste the outline so that you have two copies of the same image.
Select the image effect 'mulitply' (or something like that) on the top image and lock it.
Then colour away!