So you see in black and white until you look at your soulmate's eyes. Even if you're blind, the cells in the retina will still activate, and someone's invented a little tiny device that goes inside the eye of a blind person to alert them when their color vision has been activated.

Matt happens to look at Foggy's eyes first, and he know then that they're soulmates, but he can't bear to trap his new best friend with the Devil inside himself, so he always keeps his glasses on to make sure Foggy will never see his eyes.

Foggy finds this out when he pulls the mask off of Matt.