I don't think that's such a good idea^^;  What 
most people do is buy a piece of acetate & paint 
or draw the lines in on that...& then overlay it 
on top of the cel.  This is a much better way of 
improving the image.  Painting directly onto the 
cel lowers its value signifigantly as well^^;.  
Hope this helps!  Give me a buzz if you have any 
other questions^_^!