Unknown Colours (Destiel/Sabriel)
The Novak's and Winchester's live in a world where everything is grey, up until you meet your soulmate, at which point they'll be able to see colours. It all seems pretty simple, unless you don't know who you met.
Dean and Castiel met drunk at a party, waking up with no memory but colours everywhere. They have no idea who they met but Castiel is determined to find his soulmate. Dean however is not exactly enthusiastic about the idea of soulmates.
Sam, the 'perfect' student, gets a bit bored and ups his small pranks to starting a war with the schools renowned Trickster. Of course, Gabriel has no idea who this guy is but that doesn't stop the war between them. And so without meaning to, the two start to know each other quite well.