light and darkness are both vague as concepts, because it boils down to a “good vs evil”, or a literal narrative most of the time. Although i think the literal approach works best, because how would you be able to tell if a character is on the side of light if their colors were dark?
if you leave it up to the audience to guess, they’re going to use their personal bias to judge whether your character is light or dark, so why not just make it obvious for them, but have the “mystery” lie in whether or not the character is good or evil? 
