Could it be that he had
aniridia? (And since you shouldn't trust wikipedia too much,
here's another source)
It's an eye condition usually connected to poor vision and aversion to light. I came across the wiki article by chance and you know.... it sort of fits. When he wrote his OWLs he had his face so close to the paper that his nose almost touched it (then again, with that nose...) and he is usually associated with skulking around in the underground dungeons or at nightly patrols. We certainly never saw him sun bathing at any rate
wink
Anyway, just some speculation. It makes more sense than the romanticised "If a love interest only gazes into his eyes long enough they'll appear to be beautiful dark brown", or any such sap.