Lines can still fade with clear UV-resistant 
glass, although the fading would occur much, 
much, much more slowly than leaving the cels out 
in the open.  Just try not to have any direct 
lighting (especially sunlight) on the cels, and 
remember to replace the glass every other year.

Yellow UV-resistant glass covers a wider range of 
UV frequencies than clear UV-resistant glass and 
is thus more effective than clear glass, but 
nobody likes the way yellow glass looks.