I think someone once said part of the problem 
with the warm colors is that the sulfer in the 
yellow can alter in chemical composition over 
time and actually eat away at the lines.  I think 
the toner quality is also a big factor, as in the 
infamous fading Toei cels.  Of course, there are 
exceptions... I have a beautiful 1st ep Miaka, an 
Usagi cel (in my gallery update tommorow) and a 
Trunks that have absolutly no fading at all.  And 
they don't give any indications of being likely 
to fade... very stong, black lines.  Personally, 
cels with no fading are my favorite.  I think a 
fw cels have faded a bit since being in my 
possesion, which is a really tramatizing thing... 
haven't even been collecting a year yet :/