Purple paint is weird - scientific proof

I have a big set of Prismacolor pencils in my craft room. They sit in three cups, sorted into warm, cool and neutrals.

I also have a big skylight in that room. the room is very bright, if you leave anything printed there, it fades rapidly!

I noticed today that the pencils are fading! Well, not the pencils, just the paint on the casings of the pencils. And not all of them, just these few. All of the affected pencils are pinky-purple shades.

Since taking the photo, I've reviewed the whole set of 144. The ones that have faded are 929 rose, 931 dark purple, 956 lilac, 934 lavender, 993 hot pink, 994 process red, 1007 imperial violet, 1009 Parma violet, 1017 clay rose, 1029 mahogany red, 1038 neon pink.

I noticed 932 violet and 996 black grape are fine. As are all the other colors!

Just thought I'd record it here to see if anyone else is interested.