Advanced Recon Commando

ARC troopers were an elite variant of clone trooper in the Grand Army of the Republic during the Clone Wars, a specialized type of clone commando. ARC troopers such as Colt and CT-5555 received specialized training and often served as leaders or received challenging assignments not suitable for less specialized clones.

Behind the scenes
According to Genndy Tartakovsky on the DVD commentary for Star Wars: Clone Wars: Volume Two, whose story is now considered part of the continuity of Star Wars Legends and not canon, the design of the ARC troopers came from Hasbro. They first appeared as Advanced Recon Commandos in the Legends story Star Wars: Republic 50: The Defense of Kamino, a comic from the Star Wars: Republic series by Dark Horse Comics. In canon, they appeared in the Star Wars: The Clone Wars television series, first seen in "Clone Cadets."