With what software ?

If it's Photoshop : create a new layer on top of your line art, select the gradient tool, make colorful love with it, and when you're satisfied with your gradient, just change the type of layer until you find some that makes your gradient appear only on your line art (I can't remember the english version of Photoshop, but it should be "overlay" and the other types next to it). Works best with a pure black line art on a pure white background.



The point is to have a cup of tea in the process, otherwise it won't work.