Hi JC

You might want to e-mail me if you would like 
more tips as this thread is starting to get old 
(I generally only check the first page). But in 
answer to your question it depends on how you are 
doing the cel. If you are painting on top of the 
cel (ie the front face) it is just like painting 
on anything, you can layer colours on top of each 
other and hence cover up any minor mistakes, in 
saying this it is difficult to cover a dark 
colour with a lighter one (ie black eyelines with 
the white of the eye). If however you are 
painting on the underside of the cel (like a 
normal animation cel) you are actually painting 
in reverse and hence can not cover up mistakes 
with the next colour. I suspect you are doing the 
former as it is easier any you can make 
corrections. 

If this is the case the techniques are quite 
different. Assuming you are painting on the face 
of the cel I suggest you lay down the larger 
background colours. In the case of a persons face 
this would be the skin, and hair. Then place on 
it the whites of the eyes, followed by the pupels 
etc and finally finish off with the marker pens 
to do the highlights. If you are however painting 
on the back its the reverse. Anyway have fun and 
if you have further questions I am happy to 
answer them (however I am no professional its 
just a hobby of mine). Either way its best to e-
mail me as I will answer it quicker.

Michael