I'd laugh my ass off, if Colorado citizens can cast write-in ballots, and Trump won by an overwhelming landslide due to the write-in ballots. According to this it's possible if Trump files the paperwork: https://www.sos.state.co.us/pubs/elections/vote/writeInVoteInfo.html
According to this it's possible if Trump files the paperwork:
https://www.sos.state.co.us/pubs/elections/vote/writeInVoteInfo.html