As a fello colorodoan, and fiber optics engineer I'm qualified to help add to the other answers,

Most of the other comments are accurate about pollen, humidity, etc. A few things left out is the marijuana haze from increased outdoor consumption, an increase in air pollution due to increased vehicle usage, the mirage effect is also more prevalent in Colorado during summer because the relative temperature between the mountain peaks and the plains is relatively similar during winter (A LOT of factors in this), and if you're on the eastern plains looking at the mountains in colorado the contrast plus angle of the sun in winter makes the mountains brighter and stand out more for a lot longer in the day. During the summer if youre up early enough on a cold day in summer the mountains will stand out more than at like noon.