Re: Why does purple construction paper fade faster than red construction paper?

Normally, construction paper is colored using dyes.  There are many color 
dyes that are not "lightfast".  Lightfastness is the ability of a color to 
remain vibrant and consistent upon exposure to light, particularly 
sunlight.  Of the many red dyes in existence, the paper manufacturer may 
have used a red dye to make the purple (since purple is a secondary color 
made by mixing the two primary colors red and blue) that was different 
than the one chosen for the red construction sheet.  Red is not the only 
color of dyes susceptible to light fugitivity (fading).  In fact, there 
are many different color dyes which suffer the same fate.  Curiously, the 
blue dye used to make the purple sheet may not be lightfast either.  This 
is evident by how the purple faded... did it just become less intense?  Or 
did the hue (the actual shade or color of purple) shift to become more 
reddish?

Typically, industries who deal with dyes often switch (if they are able) 
to using pigments instead of dyes.  Pigments are much more stable, on 
average, to light exposure... This is why house paints, especially 
exterior, use pigment as the primary colorant of choice.