Clone trooper film series and in the "Star Wars" expanded universe media, a number of specialized trooper units are seen, including elite special forces units such as the 'Advanced Reconnaissance Commandos' (ARC troopers), and Null ARCs, who usually work alone. All clone commandos and ARCs were trained by Mandalorian mercenaries; the elite Clone Troopers are known as Shadow Troopers. The style and color of a clone troopers armor often varies depending on their rank, specialization, unit, or environment, for example, the clone troopers on Kashyyyk wear camouflage green scout-style armour whereas the 501st legion wears standard white armor with blue accents. Another
Star Wars: The Roleplaying Game later became the "Star Wars" expanded universe, and its sourcebooks are still frequently cited by "Star Wars" fans as reference material. Lucasfilm considered the West End Games' "Star Wars" sourcebooks so authoritative that when Timothy Zahn was hired to write what became the "Thrawn" trilogy, he was sent a box of West End Games "Star Wars" books and directed to base his novel on the background material presented within. Many of the first uses of "Star Wars" alien names (such as the Twi'lek, Rodian, and Quarren) appeared for the first time in WEG's "Star Wars" books. Even after Disney's reboot