SSD619Valkfan

My first post on MW World ever! I've got yellowing on my 1/60 Hikaru 1A as well. I noticed that the overall color scheme of subsequent valks is different than that initial 1A. All the other valks I own are grey instead of the full-on white of the Hikaru 1A and haven't yellowed at all (1A Max DYRL, 1S Roy, 1S Roy Strike, 1J FAST, 1D, 1A Cannon). Is it possible for the grey plastic to fade or yellow?