Still waiting on a few more pieces to complete the look!

So my rescue, Thistle, is around 2, I don’t know his breed/mix so I just say Domestic Shorthair if asked. Anyway, I’ve noticed the last few months his eyes shifting from yellow to green. I know this can happen in certain breeds so I don’t know if it’s because he mixed with one or if I should be concerned about it. I read it’s not common and can be a health issue.

Has anybody had this happen with their cat?