All the Gundam Wing genga I have are on white 
paper.  I think they just use whatever paper is 
on hand and that whether it's white or yellow 
doesn't make a difference.

The genga that came with that 18" gundam wing pan 
cel is two pieces of paper taped together. Now I 
need to find a cel/art book big enough for both 
the cel and genga, heh.