What would it take to make CO back to a light red state again?
It's impossible. Maybe,
just maybe, a Republican could win a single statewide race (even if it is quite unlikely) at some point in the near future, but the Democratic voter coalition that has developed in CO is too much for it to become consistently light red. Front Range liberals, ski country transplants, rising levels of non-whites, a multitude of suburban voters increasingly hostile to conservatives - there's a reason Cory Gardner's approval rating is in the 20s.