Types Of Royal Guards
Lelouch's Royal Guard
Following his rise to power near the end of the series, Lelouch used his Geass to force the service of many Royal Guards for himself, forcing them to swear absolute loyalty to him to the point of sacrificing their lives to ensure his own survival. Lelouch's Royal Guards wore grey and white uniforms.
Schneizel's Royal Guard
Schneizel's Royal Guards, who were led by Kanon Maldini, protected him and carried out his orders. They wore green suits, and occasionally wore white capes.
Cornelia's Royal Guard
Cornelia employed many Royal Guards who fought alongside her throughout the first season, such as the Glaston Knights. Her Royal Guards wore maroon uniforms and piloted Gloucesters.
The Imperial Guard
Charles' Royal Guards are seen only in the Imperial palace, serving the Emperor directly. They wore gray uniforms with red sashes and short black capes, and carried rifles with elaborate bayonets, possibly more for ceremonial purposes than for actual combat; (the Swiss Guards of our world are the personal bodyguard of the Roman Catholic Pope, and similarly carry polearms). They are connected in some fashion to the OSI, evidenced by a mention of them in one of Rolo's case files (Counterattack at the Gallows) and that they wear the same helmets. Jeremiah Gottwald was a member of the Imperial Guard (but did not wear the helmet, suggesting junior or cadet status) during his time at Aries Villa. Cornelia wore their uniform (with her trademark cape) while commanding Marianne's guards.
The Knights of Rounds
The Knights of the Round are known to have had their own personal forces. Little is known about their operation or uniforms, but they were ultimately defeated by Suzaku when they tried to attack Lelouch near the end of the series during the Knights of the Round Uprising. Whether these personal forces were legitimate Royal Guards is not explicitly stated.
Clovis's Royal Guard
Clovis' Royal Guards accompanied him to the Shinjuku Ghetto in Area 11 to try to recover C.C. after the capsule containing her was stolen by the Japanese resistance. They were the first Royal Guards to be seen in the series, and were among the only people who knew what the capsule they were sent to recover actually contained; lower-ranking soldiers and civilians were led to believe that it contained poison gas. They are shown to wear reddish-brown versions of the standard infantry uniform while officers wear flat caps and a black baldric or short shoulder cape, possibily as a symbol of rank. A group of Clovis' Royal Guards became the first people to be affected by Lelouch's Geass, and were ordered to kill themselves by him as his first step towards defeating Britannia.