I haven't heard anything before about grays being predisposed to eye issues... My gray mare has quite a few melanomas but they don't slow her down at all. The only thing I have to do different with her because of them is that she can't wear a bit because she has them in her mouth... But other than that, she's fine, melanoma-wise.
Eye-wise, she does have this "thing" where she can't see very well in dim light conditions so she's a little spookier at dawn and dusk. But, I have no idea if that's becuase of her color or her age. The vet said it was because of her age (26) and I'm inclined to believe them...
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.