Which are a shadow of things to come; but the body [is] of Christ.
Note 28 at Col 2:17: The word "shadow" means "a faint indication" (American Heritage Dictionary). When used as a verb, it means "to represent vaguely, mysteriously, or prophetically" (AHD). Each of the things listed in Col 2:16 served to represent something that the Messiah would accomplish. Now that Jesus the Messiah has come, the representation isn't necessary. The New International Version translates this verse as "These are a shadow of the things that were to come; the reality, however, is found in Christ."
A shadow is never as important as the person who casts it. Those who cling to the Old Testament shadow are missing the New Testament reality found in Jesus (see note 12 at Ro 14:14).
Note 29 at Col 2:17: Jesus Christ is the body that cast all the Old Testament shadows. Now that we have Jesus Himself, why would anyone still cling to the shadow?