In the 2018 midterms, Democrats gained control of Colorado’s Senate and elected a Democratic governor (the previous three governors were Democrats, too).
However, nearby states like Utah and Wyoming very consistently vote Republican. Why is Colorado so different than those other nearby states with a lot of rural areas?
One of the best predictors of voting patterns in the US is population density. You can see population density by state here. Although this isn't a perfect reflection of how urban a state is (a state with most of their population concentrated in cities could have the same population density as one with their population spread across their state, and the former would likely be more blue than the latter), this gives some indication. Colorado has a population density of 56 people per square mile, while Utah is slightly smaller at 39, and Wyoming has 6. Other factors include religion and how "cosmopolitan" the area is (and by that I mean interaction with other areas, such as being a tourist area).
If you take out the greater Denver area (Denver, Boulder, etc.), and tourist areas such as Aspen, Colorado is quite red. See, for instance, this graphic . The only deeply blue counties are Denver, Boulder, Pitkin (contains Aspen) and San Miguel (contains Telluride, another tourist spot). You can see the locations of these counties here. Teton County in Wyoming also was blue in 2016, likely related to it containing Yellowstone National Park. Other large blue counties Arapahoe, Adams, Jefferson, and Broomfield are in the Denver area, and Larimer contains Fort Collins, another urban area. You can see an aberration in El Paso, being both large and red. This is due to Colorado Springs, a city with military ties that has become a gathering point for conservative Christians (Ted Haggard's megachurch, for instance, is located there). Utah politics are also significantly affected by religion, with the state being dominated by Mormons.
There is a geopolitical feature called the Front Range Urban Corridor that contains such cities as Pueblo (another place that leans blue), Colorado Springs, Denver, Boulder, Fort Collins, and Cheyenne in Wyoming. Running along the eastern edge of the Rockies, it is significantly more urban and Democratic than most of the central US. Most of the support for the Democratic party in Colorado comes from people in this corridor.
Clinton won only a plurality of Colorado votes, 48% to 43%. If we were to take out Denver county votes, the plurality of the rest of the state would go to Trump, 47% to 45%. Take out Denver and Boulder, and Trump would almost get a majority, 49% to 42%. Take out the FRUC counties of Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, El Paso, Jefferson, Larimer, Pitkin, Pueblo, San Miguel, and Weld, and Trump would have won 51% of the Colorado vote to Clinton's 40%.
One very simple answer is that Colorado has a higher percentage of its population living in urban areas. Census data puts Colorado as 86% "urban" while Wyoming is only 65% urban. Likewise nearby states like Montana, Kansas, and Nebraska are 56%, 74%, and 73% respectively. Urban areas tend to vote for Democrats more than rural and suburban ones.
Utah is a special case, being shaped by its more conservative Mormon religious culture, so despite being more urban with the dominance of Salt Lake City in the demographics, it consistently sends Republicans to Washington.
Note that this doesn't fully explain the complex politics of Colorado. There's a lot going on in the Republican party right now. Trump brought a realignment away from the classical liberal/religious morality/constitutional restraint focused GOP towards a more nationalistic, populist, economically protectionist party. This shift might have pushed the more libertarian Colorado voters towards the Democratic party.
First, New Mexico (Hispanic/Indian) and Utah (Mormon) are both culturally unique and distinct from the rest of the Rocky Mtn West. 100 years ago, Colorado, Wyoming and Montana were all very similar - overwhelmingly white, rural and conservative with a limited populist/progressive bent. Wyoming after all was the 1st state to extend the vote to women and in general these states have had a libertarian ethos as opposed to a religious political environment.
The fundamental difference is Denver. While Wyoming and Montana remain relatively insular states, the ambitious (and corrupt) leadership of Denver more than a century ago decided to grow the state beyond it's historic agricultural and extraction roots. It took a long time but Colorado has changed profoundly from its cousins in Wyoming and Montana because of these decisions. Today, ranching and mining are modest economic contributors to the state with tourism and services being the leading elements. In a sense, Colorado has followed the California path just 75 years behind while Wyoming in particular chose to stay as it ever was.
The population base is located predominantly along the Front Range stretching from Pueblo in the south to Fort Collins in the north. Outside the Front Range, the political culture is almost uniformly rural and conservative with the minor exception of some of the mountain towns such as Aspen. However, knowledge-economy of the front range is much more diverse with a much higher percentage of lifestyle liberals, hence the politics. It's not blue but blue-purple and that's a relatively recent phenomena but likely amplifying trend. It's not the Bay area but it's much more progressive than any of the surrounding states including Kansas/Nebraska to the east, Utah to the west and traditional Wyoming to the north. Utah's Salt Lake City is in some respects similar to Denver but the progressive element is diluted by strong Mormon presence in the rest of the State. New Mexico is also somewhat similar but given it's relative poverty and distinctive cultural base is not a direct comparison.
What most people aren't considering is the education levels of the urban areas vs the rural areas. You do not have very many college educated people in rural areas.
Most (but not all) of the split can be attributed to education levels. In my personal experience the more educated the person the more likely they are to be aware of the complexities of issues and "usually" lean toward liberalism.
Most of the rural population have a H.S. education or less, but they don't NEED anymore than that as their industries generally run towards agriculture and industrial fields.
I would have made it a comment to carsontools answer, but...
While education may not have a causal relationship (or at least not one that would be recognized by a statistician) concerning political choice, it is hard to disregard the data.
Compare the group of states at the bottom of education attainment rankings. The rankings are available via WalletHub. Ranking and policital choice by presidential election(last 3 elections).
West Virginia 50 R Mississippi 49 R Louisiana 48 R Arkansas 47 R Kentucky 46 R I would consider all of these rural/agricultural states.
Now compare the top group
Massachusetts 1 D Colorado 2 D Maryland 3 D Connecticut 4 D New Hampshire 5 D A couple maybe classified as rural but none that are agricultural.
That was to get a baseline. Now for the states around Colorado. They are all over the place.
Utah 11 R Wyoming 21 R Nebraska 19 R Kansas 15 R Oklahoma 41 R New Mexico 38 D
And that is why a statistician would say there is no causal relationship between education and political choice. That being said, I would still say it has strong bearing considering the top 10 are blue and bottom 10(outside of NV) are red. As far density goes, it may not have strong bearing, but it definitely has some.
