The Rapids (2-0) played short-handed for more than 36 minutes and continued their strong play after a 2-0 win over Columbus in the opener.
Castrillon put the Rapids ahead 1-0 in the 56th minute when he corralled a loose ball in front of the net and tapped the ball past Philadelphia goalkeeper Zac MacMath.
Omar Cummings, who picked up an assist, sent the crossing pass toward MacMath, but the ball slipped through his fingers.
Colorado kept the pressure on after midfielder Jeff Larentowicz received a red card in the 58th minute.
In the 62nd minute, Cascio's goal extended the Rapids' advantage.
Forward Lionard Pajoy scored in the 67th minute on a header.
The Union (0-2) began their third season following a trip to the Eastern Conference semifinals. Though Philadelphia lost to the Houston Dynamo in the two-match series, the front office brought in 14 new players this season.
So far, it's been a struggle.
Philadelphia lost 3-1 at Portland last Monday and managed just one goal again.
The Rapids remained unbeaten all time (3-0-2) against the Union.
(Copyright 2011 Philadelphia Union. Reprinted by permission)