Colorado optioned Castano to Triple-A Albuquerque on Tuesday.
Castano made five appearances with the big-league club after being called up Sept. 1. Over 6.2 innings, the 28-year-old posted a 2.70 ERA, 1.20 WHIP and 4:3 K:BB. The Rockies reinstated Kyle Freeland (neck), leading to the team's removal of Castano from the active roster.