Blue and Green
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.
In June 2019, Stands launched a collection for Pride which i a Destiel t-shirt which featured a cannon (aka canon) with rainbow coloured feathers shooting out of it. The accompanying text read "You’ve been patient and subtle about your desires for years, so we’re taking this matter into our own hands and making Destiel cannon! We’re not sure why you’re so into civil war re-enactments, but we have a feeling the Destiel Cannon shirt will be your OTP (one tee, perfect). It’s emblazoned with a cannon designed in the bluest blue to ever blue and sparkling green, so it’s sure to be right up your alley...The fabulous rainbow feathers are proof that the cannon will go off (but never prematurely)."