Went to the library to do legitimate research and 
stopped by the art floor and read everything they 
had on cels & cel animation. Ok, so they weren't 
very specific about anything at all, outside what 
the steps are called (inking, oqaquing, etc) but 
I did learn a few things:

Disney's 'colorform' cels.  Those cels that were 
cut up and put on complementing backgrounds?  
That was more because the studio was looking for 
work for people it didn't want to lay off in the 
off-season than for aesthetic purposes. And LOOK 
what it started...

Blank cels- You're only supposed to have 4 or 5 
layers of celulose TOPS in any scene because the 
colors will become too faded on lower layers (and 
they actually have tones for each color that you 
put on different layers so they film equally) but 
if you have say 4 layers at any time in a scene 
you have to have 4 layers throughout or it will 
look off (the background will change brightness 
or something like that) so the extra blank layer 
just holds the place where a layer with paint is 
in one of the frames had a layer so everything 
stays in balance.