*grrr* I wrote a perfectly wonderful answer and 
then I forgot my password to login so it was 
lost. I'll rectify that soon (I mean, how many 
passwords can I remember at once -- I'm a moron)

Anyway, according to Brian's animation production 
information located here:

http://www.kimonocels.com/Animation/production.htm
l

Yellow paper usually indicates concept art, not 
stock footage. Different colored paper also means 
that the sketch is in a different stage of the 
animation process. You'll probably notice that 
the yellow sketch is rougher and probably doesn't 
have a sequence number. You can see an example 
here:

http://members.fortunecity.com/wndrkn/kunzite.html

You'll see that the first genga of Kunzite is 
rough and of the same sequence as the second, 
more complete sketch (on white) that follows. The 
last is also on yellow paper and yet has no 
sequence number (although obviously produced for 
the same cel).

Here's another example:

http://members.fortunecity.com/wndrkn/genga2.html

and a concept sketch of Nuriko from Fushigi Yuugi:

http://members.fortunecity.com/wndrkn/nor3.jpg

I highly recommend Brian's site -- he explains 
the differences between sketches (layout, genga, 
douga) as well as cels. It's extremely handy and 
provides lots of examples. You can find a direct 
link to his page from the main cels.org page.

One caveat: studios may do things different from 
one another. I haven't seen enough examples to be 
able to say for sure that this is the case for 
everybody. I'd love to see any more genga 
sequences that anybody has -- I simply love 
them!!!!

Sorry for the long post -- thanks for your 
patience and hope this helps!

Wendy aka wndrkn aka the idiot who can't remember 
her password