Moses Basket Mattress

Hello,

Can someone tell me why the mattress in a moses basket changed colour over time.

I have a moses basket which i have been using for about a month now and I have noticed that the mattress has started to turn a little yellow. I also have a friend whose cot mattress also went yellow.

The staining isn't from the baby and it is a foam mattress. I'm just a bit puzzled about this and I'm wondering if it is safe to keep my baby in the moses basket. I bought it from Asda so I asked them about it but they don't have any idea. They just say thats normal, but I want to know why.