After 1931 graduation from the Naval Academy, Colwell served in the battleship USS Maryland (BB-46), on the Battle Force staff in the USS California (BB-44), and in the four-stack destroyers USS Rathburne (DD-113) and USS Aaron Ward (DD-132). He did postgraduate work in ordnance engineering and a 1939-42 tour in the battleship USS Idaho (BB-42). In 1942 was gunnery officer in Halsey's South Pacific Force; later CO of the USS Converse (DD-509), making forays towards Truk as a part of Admiral Burke's destroyer squadron, "Little Beavers." Long tour at the Naval Proving Ground at Dahlgren, then was XO/CO of the USS Missouri (BB-63), on CinCPacFlt staff, and in the Bureau of Ordnance. In 1953-54 he was EA to Deputy Secretary of Defense Roger Kyes. A high point of his career was serving as deputy to Admiral Red Raborn during the development of Polaris. Was CO of the oiler USS Elokomin (AO-55) and USS Galveston (CLG-3), first guided missile light cruiser. As a flag officer, Colwell was Commander Amphibious Group Four; Director of CNO's Long Range Objective Group; Commander Amphibious Force Pacific Fleet; and Deputy Chief of Naval Operations (Fleet Operations and Readiness). He retired in 1969.