The PAC10 was looking at up to 6 Big 12 teams - Texas, Texas A&M, Texas Tech, Oklahoma, Oklahoma St, Colorado.
Colorado joined while the status of Texas/Oklahoma etc was still up in the air. So they weren't a fallback option. It may have been that they were seen as a small defection that could start the avalanche though.
Utah was definitely a fallback, the quick pivot after Texas/Oklahoma etc chose to stay in the Big 12 - Utah joined 2 days after the others declined to do so.