Petit (1-0) allowed two runs on six hits and three walks, and Hunter Pence sparked the offense with a two-run homer.
Petit, who won his last start while with Arizona, labored in the Mexican League and the triple-AAA farms for Oakland and the Giants. He was staked to a 3-0 in the first inning and held Colorado to single runs in the third and fourth before giving way to the bullpen. Sergio Romo earned the save.
Charlie Blackmon, Wilin Rosario and DJ LeMahieu each had an RBI for the Rockies. Chad Bettis (0-3) surrendered four runs on nine hits and a walk over six frames in the loss.