Colonel was a rank that was common to most militaries throughout the galaxy. It was generally below the rank of general and above the ranks of major and commander. The rank itself was relatively high in the greater chain of command.
Erich S. Datoo, a Colonel in the First Order
The rank of colonel was used in militaries during wartime at least as far back as before the outbreak of the Clone Wars. During that particular war, there were colonels on both sides of the conflict, the factions being the Confederacy of Independent Systems and the Grand Army of the Republic. Colonels were not generally known to participate in combat, instead serving as strategists and high commanders. One such tactician was Meebur Gascon, a Zilkin officer known for his skills as a commander. During the war, he commanded the droid unit D-Squad on a mission to steal a Separatist encryption module.
The rank was still in use during the reign of the Galactic Empire, held by Imperial officers in the Imperial Army. It was also a non-martial rank as well, being a senior rank in the Imperial Security Bureau. One such ISB colonel was former Republic admiral Wullf Yularen.