Hello,

Well this is my take on it.

The top right and middle left are Genga,
Drawn by the Genga man for that episode.

The yellow ones are Shusei Genga (retouch)
and because it is retouch only the parts
that need retouching are redrawn.

The retouching is normally done by some one 
higher
up to the food chain so _some_times_ the
Shusei Genga can be a greater prize to have.

If you can read Kanji watch the credits as
it normally lists who did what for the episode,
Or by a Book with an episode list in it.

>A Genga is a drawing done by a higher up
>animator  to show certain aspects of the
>characters for that scene. It's a loose
>guideline for the animators, and one Genga
>can be used for a good chunk of a scene

Yes this is correct :) and what you have
explained is what is here....

http://www-personal.umich.edu/~tdkim/cel/
relena4-gundamW-A1END-gengas.jpg

This set up looks like a A-1 Tome set up, The
only thing that is going to move is the mouth.
As you can see the number on the mouth is C-
3,
So the not all the frames have been drawn. 
Unless
tk hasn't scanned the others that is ^_^;

Also it has the numbers on it A1, C1 etc. etc. 
This
is a pretty good sign , As I explained before 
"Key
Cel" in Japanese is "Genga Number."

This is a pretty tough subject to write on. 
Some
studios do things differently , I don't think
Ghibli uses yellow sheets... Hmmm Can't
remember yellow sheets being attached to
Ghibli Genga, Maybe those guys draw 
perfectly
all the time :)
Also some Genga like the ones I am
sorting today don't have numbers on them :(... 
Strange

and I can't always tell anyway  :-) 

Stephen