Monster Zero x SciFi Japan - Archive Only

I've had the Bullmark Gorosaurus on the left for awhile. Obviously it has a ton of paint wear, so I just purchased the one on the right to have a nicer version. When comparing the two though, its quite amazing how different the base color is on the left when compared to the normal version on the right. Is the one on the left simply discolored or are there other base colors for Gorosaurus besides the typical light blue? In Sean's Linkenback's guide it does show a Gorosaurus with a similar off colored base next to the striped belly Gorosaurus. But the book doesn't mention two variants nor does Clubtokyo list different ones either. Just curious if anyone has some insight on this matter?

Thank you for your help Legion! Sorry for not getting back sooner, been rather busy lately. I think sun damage is the verdict. I didn't pop the head off but when I took a pic of the feet with the flash on, you can see some light blue in the tail joint.