Colorado (11-14-4) can make the playoffs with a win in its next match -- against Real Salt Lake Saturday. Chivas USA (12-11-6), has already clinched a playoff berth.
Clark opened the scoring with a header into the side netting in the 67th minute.
Chivas USA's Jonathan Bornstein tied the contest less than 5 minutes later
Casey hit the match-winner, getting the ball in the penalty area and firing a shot past Colorado goalkeeper Bouna Coundoul with 5 minutes left on the clock.
Chivas plays Houston Saturday, ending its regular-season schedule.