DD Snow Miku Default shoes yellowing?

Hi everyone,

 

Today, I went to check my DD snow miku shoes that I always keep in her dollfie box. When I look on it, I notice some of yellowing in her shoes. I only use it once and never use it again around 1,5 years ago and store it like a new one in plastic and silica gel in her box.

I want to now if this shoes need another different maintenance and storage? I also want to ask is there any way to remove the yellowing in snow miku shoes?

 

Thank you in advance.