Blue and Green
From Super-wiki
The colours Blue and Green have become the official colours of the Destiel fandom. Their use arises from the eye colour of Castiel (blue) and Dean Winchester (green). While the characters' eye colour was often remarked upon by fans and included in fanworks it wasn't until around 2015 that the colours started to strongly associated with the fans of Destiel.
The adoption may have been inspired by fans who ship Harry Styles and Louis Tomlinson from the band One Direction. Harry has green eyes, and Louis blue, and those colours were also used to identify their microphones on stage. Larry fans often used blue and green hearts in their posts and for fandom projects.