So, looking at old photos has informed me that I'm not misremembering, and that my eyes used to be flat green where there's now a burst of bronze at the centre.
The internet reckons that this likely to be acquired central heterochromia and that it can be a symptom of something.
Fortunately the main candidate is an eye infection, and I had a really awful one a couple of years ago, but I suppose I should still take this to an opthalmologist as it can rarely be an early warning sign for glaucoma, nerve damage and other things that require preventative treatment.