I agree.. doing a sample test with cheapies would 
be interesting.

The thing I've noticed with mine... I've been 
collecting for a little more than a year from 
this particular series, but I didn't put them in 
new bags until 6 months ago.  And it was probably 
a month - month and a half when I noticed the 
fading.  I keep them in cel books in the bottom 
my closet, so it's not a case of light/heat 
exposure.  I'm not sure... ^_^; 

Could just be the paint color itself.  It only 
started to on cels with darker shades than 
without, so I could just be overly paranoid of 
something that's inevitable anyway.