SpyridonThe Scooby farthest in the picture should have a blue collar.
Though, Scooby never wore a red collar in the older episodes. Because their fashion styles were referencing to older Scooby Doo episodes, whereas Shaggy's came from the 13 Ghosts of Scooby Doo and Fred, Velma, and Daphne's came after that. But I get it, they need to be able to tell the 2 Scoobys apart. Lol.