Yep, pinks and purples are the worst for fading!! Any bright light is bad (not just sunlight). I always put my fondant in a dark cupboard or in your case, I guess a box over the cake might have helped.
I wonder, are you using purchased fondant? Are the colors even right when you tint the fondant?
Purchased fondant typically has citric acid in it. It reacts with the purple and the pink and dulls them out. It's not really fading. Think of when you color gumpaste lavendar or pink, those don't "fade" right? SOme brands of purchased fondant are worse than others for "acid" content.
I suggest if you need lavender or pink fondant, make homemade fondant or airbrush the color. Sometimes, if there is too much cream of tartar in my IMBC, the same thing happens. Also, I've noticed Wilton pink will crap out more than Americolor.
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