The references both to colder air holding less water vapor, and relative humidity, might be confusing. The water vapor, when it stays in the form of water vapor, has minimal impacts on visibility. A larger impact, captured in the equations above, occurs when the water vapor combines with pollution in the air, specifically ammonium nitrate and ammonium sulfate in those equations. Those are hygroscopic salts that will become hydrated if the relative humidity is high enough, forming larger particulates that will scatter light more, especially if they start to clump together as they can do. Whether they become hydrated is controlled by the relative humidity, not the absolute humidity.

For more on this, see the EPA's guidebook on air quality impacts on visibility, written with an emphasis on views in national parks. https://www.epa.gov/sites/default/files/2016-07/documents/introvis.pdf Section 4 is the most directly relevant, particularly starting on p. 22 (p. 31 of the pdf).

The relative humidity in Colorado tends to be low in both summer and winter, and also varies greatly with time of day, so I'm not sure to what extent humidity explains what OP is seeing.

Edit: the monthly average RH in Limon CO (on the plains, east of the mountains) is a little bit higher in the winter than in the summer, but the swing in relative humidity over the course of a single day is often much larger than that seasonal variation.