What do you want the finished product to look like? Different mediums have different looks. Also, the type of paper and ink you used may be a deciding factor on what mediums will be the best to color with. You may want to do a test on a separate sheet to ensure that the coloring medium will work on top of what you've already got and not mess it up. Might also be a good idea to make a photocopy or scan and do a test run of color options before working on the original.