Some of the dyes used to color things are fugitive. They tend to change with exposure to light (fluorescent light especially or sunlight).
The color changes you describe are common with some colored pencils that are not lightfast. Yellows and reds(esp pinks) are the most affected. (they are often mixed into various "brown" shades.) Drawings/paintings done with non-lightfast pencils or paints (many art supplies coming out of China like Marie's brand for example) fade very quickly.
This may not be a factor for you, but it is worth considering that the environment may not be as stable as you think.
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All truths are easy to understand once they are discovered; the point is to discover them.-- Galileo Galilei
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Weezie looking at petunias 8.25.2017
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