I read on another post that Colorado zone 5 may still receive frosty nights. Wait until April.

Yes, unless you can cover them on frosty nights wait until mid-late April.

Last year I lost daylilies planted in the last week of April due to frost. It went down to 22 degrees the day after I planted them and it was so windy that I couldn't keep any cover on them. 3 of 7 piddled away and finally died from the stress. Iris are tougher than daylilies but I'd still be worried they would get bit pretty hard if their roots are not deeply settled in. I'm worried about everything outside that has been awakened by our recent warm weather.

Only if you can plant them w/o disturbing the roots at all & cover them again to protect from frost would I put them out now. Or you could try using Wall-O-Waters for protection.

I hear we're bracing for snow again on the Front Range this Monday.