After spotting the Rapids a 1-0 lead early in the second half, Castillo outran the defense in the 60th minute and sent a cross into the box, which Carlos Rodriguez converted into a goal at the back post.
Then in the 81st minute, Rodriguez came down the right side and sent the ball to the end line, where Castillo picked it up and fired it in for the decisive tally.
The victory allowed Dallas (4-9-7) to break a 14-match winless streak.
Omar Cummings scored for Colorado (7-11-1).
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