Is it both eyes or just the one? Is it a sudden change, as in ok yesterday but different today - could indicate an injury.
Have you changed any products eg hay, bedding, washing powder for fleeced etc? Could it be an allergic reaction?
Does your piggy object if you touch around the eye/mouth area? Dental issues can sometimes cause eyes to be affected. Is it a bloodshot looking reddy brown?
Not sure if their eyes change colour with age, but would think that would be a gradual process if they do.
Only two of mine have ever really showed the white bits clearly, and I'm pretty sure that they alwaus stayed white though.
Could give them a bathe with cooled,boiled water with a tiny pinch of salt in. Might help reduce colour change & soothe it if it's an irritant.
Hope you can sort the issue, hopefully it's nothin drastic if piggies still acting normally ,keep us updated I'm Intrigued as to the answer.
Might be worth phoning the vet, they can discuss any suggestions with you, might even be a simple explain to it & they can put your mind at rest

Xx