What would it take to make CO back to a light red state again?
The last time Republicans won CO, they won the suburban Denver counties of Jefferson, Arapahoe, and Broomfield. In fact, those counties (or their equivalent areas) used to vote R for many decades (the only time they went D between 1940 and 2004 was in 1964). If the Republicans can find a way to flip these counties back, then they can probably win back CO.
They don't necessarily have to win them. But they have to keep them very close like Gardner did.
https://uselectionatlas.org/RESULTS/state.php?fips=8&year=2014&f=0&off=3&elect=0&class=2
I think that a result of Udall not taking the race seriously more so than Gardner doing anything special
Udall did take Gardner seriously, he just didn't know how to run a campaign, lol. It was actually fairly surprising the race was within 2 points when all was said and done. I think most people expected Gardner to win by a far bigger margin considering Udall basically became a punchline by the end.
But yeah, I'm just talking geography here. He proved Republicans don't need to actually win any of those counties to win the state overall, they just need to keep them somewhat close, something that I highly doubt Trump could do.