|MadSci Network: Chemistry|
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.
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