Why is Colonel Yorke called "Colonel Yorke"?

You may ask is this some sort of trick question? Isn't it normal to call a colonel a colonel?

Yes it its normal to call a colonel a colonel.

Yorke is seen wearing the leaf insignia of either a major or a lieutenant colonel in several scenes.

So if Yorke wears leaf insignia and is called "Colonel Yorke" he must be a lieutenant colonel, who would thus be called "Colonel" for short. Right? Right.

Except that in 1879 or 1880, the fictional date of Rio Grande 15 years after Sheridan was in the Shenandoah Valley, most majors in the United States Army were called colonels or generals, and most lieutenant colonels and colonels were called generals.

The federal government of the United States had several different armies in the Rebellion of 1861-65. There was the United States Army, (abbreviated USA) which was the standing army, the United States Colored Troops (USCT), and the United States Volunteers, (USV). The loyal states and territories raised volunteers and organized them into regiments of state or territorial volunteers which were mustered into federal service and paid and supplied by the federal government.

There were only about 16,000 officers and men in the regular army at the start of the Civil War in 1861, and by May, 1865 there were about 1,000,000 soldiers in the armies of the United States, an increase of about 62.5 times. So the armies needed about 62.5 times as many officers as before, mostly from civilian life. And many officers in the regular army took leave of absence from the regular army to serve in higher ranks in the United States Volunteers.

There were two types of ranks in both the USV and the USA. There were substantive ranks and brevet ranks. Many people confuse brevet ranks with ranks in the USV. Officers in the USV, like officers in the USA, had substantive ranks that they were paid in and commanded in. And officers could also be commissioned in brevet ranks in both the USV and the USA.

What is a brevet rank? That is a rather complicated subject, but to oversimplify brevet ranks were mostly honorary with little power. An entire book was published on the subject, The History and Legal Effects of Brevets in the Armies of Great Britain and the United States, From Their Origin in 1692 to the Present Time, by Brevet Major General James Barnet Fry, 1877, which can be read online here:

https://archive.org/stream/historyandlegal00frygoog#page/n16

As an example, George Crook (1828-1890) was a regular army officer who became a brigadier general of United States Volunteers (USV) 7 September 1862, and a major general USV 21 October 1864, and was mustered out of United States Volunteers 15 January 1866 while a captain in the regular army (USA). He was promoted to major 18 July and lieutenant colonel 28 July 1866, brigadier general USA 29 October 1873, and major general USA 6 April 1888. He was also awarded 6 brevet ranks, including brigadier general and major general USA and Major general USV.

https://archive.org/stream/historicalregis03heitgoog#page/n323

The privileges of brevet rank included being addressed by that rank and wearing the uniform of that rank while off duty. Since Lieutenant Colonel George A. Custer of the 7th Cavalry was a brevet brigadier general and major general he was known as Brevet Major General Custer or informally as General Custer.

Other 7th Cavalry officers with brevets in 1876 were Colonel (Brevet Major General) Samuel Sturgis, Major (Brevet Lt. Col.) Joseph Tilford, Major (Brevet Col.) Lewis Merrill, and Major (Brevet Col.) Marcus Reno. Lower ranking 7th cavalry officers with brevets included Captain Frederick Benteen (Brevet Col.) and Captain Thomas Custer (Brevet Lt. Col.).

It is easy to imagine that Yorke might have been a general of USV during the Civil War. John Wayne was 42 or 43 when Rio Grande was made, which would make Kirby Yorke born about 1836-38, though he could be a few years older than John Wayne. Civil War major generals included Wesley Merritt (b. 1834), Nelson A. Miles (b. 1839), George Custer (b. 1839), and young brigadier generals included Emery Upton (b.1839), Ranald Mackenzie (b. 1840), Chareles Cleveland Dodge (b. 1841) and Galusha Pennypacker (allegedly born 1842 or 1844).

There were about 554 to 564 men appointed as substantive rank generals in the USV or USA during the Civil War. But a lot more than 564 officers - 1,600 according to one source - were awarded brevet general ranks for the Civil War. So it is a little surprising that Kirby Yorke was not a brevet general and called "General Yorke". (It is possible that he was substantive major and a brevet lieutenant colonel).