Sometimes believers can behave contrary to what they say they believe. In our flesh, we don't always see life as clearly and accurately as we should. In these verses Paul clarifies such an inconsistency in the Colossians' lives.
Freedom (20): Paul raises a rhetorical question in 2:20, "Why … do you submit yourselves to decrees such as…?" Since there is no basis for obeying the rules that come from the world's system of belief, believers should not live under the rules of this age, whether secular or pagan or syncretistic. We must not behave as though we still live in that world. We are free from false worldviews and their obligations. Instead, we serve a different Master (2:6)!
The Rules (21-23): Paul then exposes the true nature of these rules. First, they are temporary: "which all refer to things destined to perish with use." They relate to this life only and have no eternal value. They're also man-made ("commandments and teachings of men") bearing no inherent authority. They deceptively have “the appearance of wisdom." The world packages its rules with an alluring façade of intelligence and shrewdness. But true wisdom is found in Christ (2:3)! Lastly, the rules of this world are useless ("no value").
 NKJV's "yourselves" is an accurate reflection of the original rather than the NASB's singular "yourself."
 We are called to submit to governing authorities so long as they uphold God's justice (Rom. 13:1-6, esp. 3-4).
 This freedom is depicted as death, unrelated to the deadness mentioned in 2:13 (the words are unrelated in Greek). Paul affirms they are free from that worldview, described as, literally, the cosmos (κόσμος).
 The antecedent of "Which all" (2:22) is the decrees listed in 2:21. Verse 22 begins an evaluation of the rules and it continues into verse 23, so that it reads, "These [rules] are matters…" or "Such regulations…" (NIV).
 Contra this is 3:1ff. where we are fixed on heavenly things, future glory and likeness to God (3:1, 4, 10).
 Cf. 1 Cor. 1:24, 30-31. This wisdom came to us via the apostles (1 Cor. 2:6, 10, 16), via the NT Scriptures.
 Uselessness is the main point. The meaning of that phrase is hard to grasp, as "against" is the preposition "toward [pros (πρός)]"), as in "result in" (NET). Also, "indulgence" (negative) may be "satisfaction" (positive).