From Halopedia, the Halo wiki
Colonel, abbreviated to COL, is a commissioned officer rank of O-6 in the United Nations Space Command Unified Ground Command (UNICOM) rank structure. It is preceded by Lieutenant Colonel, and subordinate to a Brigadier General.
In a draft version of the UNSC Pillar of Autumn page in Halo: Warfleet – An Illustrated Guide to the Spacecraft of Halo a Colonel Barry Bizzell was mentioned, said to be the commanding officer of the 3rd Battalion 67th Marines. Ultimately, Col. Bizzell, and the battalion he was said to have commanded, were cut from the final published version of Halo: Warfleet.