Board Thread:General Discussion/@comment-24012316-20140813040759/@comment-26808654-20140813044802

Why Sonic's eyes went from black to green: the design changed to make Sonic look more humanoid, simple as that. Black eyes would've looked really weird so they gave him eye colorations.

Why green?: Why not? Why are your eyes whatever color?

Sadly there's no great theory or amazing mystery or grand secret, it's as simple as that. However, before you ask, the emeralds would have nothing to do with it. Sure they have tons of power but they aren't radioactive. Constant usage of them wouldn't alter ones appearance. They are gems of miracles that turn your thoughts into power and when you collect all 7 a miracle happens. I doubt any of Sonic's thoughts would change his eye color.

If you really want some sort of theory though, maybe hedgehogs just have those kind of genetics. Maybe once they reach adolescence their eye coloration begins to show. Sonic's speed is a genetic thing so why not his eyes?