Castano was optioned to Triple-A Albuquerque after Sunday's doubleheader sweep of the Mets.
Castano was called up to serve as Colorado's 27th man ahead of Sunday's pair of contests in New York, but he didn't appear in either victory. The right-hander will now return to the Triple-A level, where he's posted a 1.23 ERA and 0.68 WHIP with five strikeouts over seven appearances with Triple-A Tacoma and Triple-A Albuquerque so far this year.