Our friends over at Star Wars News Net made us aware of the tweet embedded below from account holder @LiamLikesSW that contains a screen shot taken of the official StarWars.com Databank entry for the Colossus from Star Wars Resistance revealing the name of the water planet on which the station is located. The planet is called Castilon and it can be found along the Outer Rim.
Thanks to SWNN and @LiamLikesSW for the info!
Located on the Outer Rim water planet of Castilon, the Colossus is a dangerous place that tends to attract well-meaning outcasts and the more unsavory denizens of far-flung planets alike. Protected by the Aces who try to best one another in entertaining and dangerous sky races in between their more serious assignments, for now the platform is insulated from the larger conflict brewing in the galaxy.
— LiamLikesSW (@LiamLikesSW) September 27, 2018