I don't think the GOP nominee can win without Colorado.
I know Hillary's weak there, but unless it's Paul (or according to this Walker), I don't expect it to be particularly viable in a Presidential year. We got very lucky in the midterms just to eek it out. I'm much more confident in Iowa and Virginia at the end of the day (and only one of those may be necessary).
Christie could possibly win it to but it'd require some serious rehabilitation of his image. Especially there with shaking off any remnants of a crusader against drug crimes or about treating the state "not well".
