There is such a thing as transparent paint. It is 
something Disney came up with to do some of the 
scenes in the original Fantasia, and the fins of 
the goldfish, Cleo, from Pinocchio. One of my 
reference books (_The Illusion of Life: Disney 
Animation_ by Frank Thomas and Ollie Johnston) 
has this to say about the transparent paint:

"The transparent paint that produced the appealing
filmy effect on the screen was made from the bile 
of an Asian ox, and was smelly and unpleasant to 
use. When this paint was employed for shadows, it 
was imperative that no outline be seen, so the 
inkers had to trace the drawings onto the cels in 
what amounted to invisible ink." (Pages 278-9, 
quoted wihtout permission.)

I get the feeling that the poor ink and paint 
girls would have *adored* to have one of our 
modern airbrushing gizmos. ^_^;;   Of course, I'm 
glad they didn't have it, since one of the pieces 
done with the transparent paints was the "Arabian 
Fish Dance" in Fantasia - in my opinion one of 
the single most stunning pieces of animation art 
ever produced *ANYWHERE*. If done again, I'm 
convinced it couldn't come out even half as well 
with airbrushing.

However, for a fancel. . . Get the airbrusher. ^_^
Asian ox bile is probably tough to come by. ^_~; 
(Not to mention that even Disney doesn't use it 
anymore, as I understand. . . But then animation 
art has gone **SO** downhill since Fantasia was 
made that you could almost cry. ~_~)

Many Sharp Smiles,
--Drac