Glossy poster is turning purple around the edges. Why?? (Photo included)?

I got an Uncharted.videogame poster over the summer, so I've had it for a while now. It's glossy and I noticed that maybe a few months after I got it, the boarder of the poster started turning a purplish color. It makes the poster look faded and the purple covers up the picture along the boarder. Is there any reason for why this is happening and how I can prevent it and make the poster got back to normal? Does the fact that it's glossy have anything to do with it? And would lighting do anything? I have the poster in an area where there's no lamps, but there's natural light because a window is sort of diagonal from it. But I don't always have it so the sunlight comes in the room. So sometimes my room is dark. And when I have a light on it doesn't really light up the area the poster is in. I also got another glossy poster from the same place I got the Uncharted poster and it seems like it's starting to turn purple along the edges as well. It's not bad yet. But I don't want it to get like the Uncharted one. I heard that glossy posters are good for most lighting conditions and that matte posters are better for direct light. I have 4 glossy posters, the rest are matte, and 2 of the glossy posters are perfectly fine. The other 2 are turning purple or are starting to along the edges. So I'd just like to know why and if maybe it has something to do with how much light they're given, which is why I described the lighting conditions of the uncharted poster. Thank you.