Before we pick up speed, I think it worth a leisurely look at the recipients (1:2) — "to the holy and faithful brothers in Christ at Colosse: grace to you, and peace, from God our Father."
Note first what Paul called them (1:2a): holy, and faithful.
It is pretty remarkable that Paul would call this largely-Gentile church "holy...brothers in Christ." What is the root idea of holiness? The root idea of creaturely holiness is not primarily behavior. Rather, the root idea of creaturely holiness is being set apart to God's ownership and His service. To be holy is to belong to God, to be uniquely set apart to Him.
This is a pretty remarkable assertion. What had Adam made us? Unholy! Sinners, by nature and by choice. As Romans 5:12 says, "For this reason, just as through one man sin entered into the world, and through sin death, and thus death came through to all men, because all sinned──..."
But then see what God declares us: positionally holy, by virtue of the person and work of Christ. Hebrews 10:10 says, "And by that will we have been sanctified through the offering of the body of Jesus Christ once for all." The Greek wording is hard to capture without an over-long paraphrase. The idea is something like "we are currently people who have been abidingly sanctified by means of the offering of the body of Jesus Christ, once for all." The latter is a time-note: it was a final, definitive act, neither permitting nor requiring repetition.
So we believers have been set apart for God's ownership and service, not by virtue of one thing we have ever done, but by virtue of the one thing Jesus Christ did. In fact, it was quite in the face of everything we had done, in spite of our many crimes against God and man. The cure could not be found in us, for we were the source of the problem. The cure came from without, by the intervention of Christ.
And then, having declared us positionally holy, God makes us personally holy. We read it in Hebrews 12:10 — "For, on the one hand, [our earthly fathers] were disciplining us for a few days in accord with what seemed best to them; on the other hand, He disciplines us for our common good, that we might be sharing His holiness." This aspect of holiness is a matter of progressively becoming in practice what we already are in Christ.
The aspect Paul had in mind here is positional holiness. It is an assertion he can make about every genuine Christian there, regardless of how they're doing that particular day. All of the other five uses in Colossians are positional (1:4, 12, 22, 26; 3:12). So Paul is addressing them first by what they all are: holy brothers in Christ.
Now, it is doubtless true that some have only claimed faith and are not holy in Christ. But Paul makes the judgment of Christian charity, and addresses himself to them as a church of professed believers in Christ who have indeed shown signs of genuine faith.
Then Paul calls them "Faithful brothers in Christ." The word pistos means "reliable, trustworthy." It is used of good pastor Epaphras in 1:7. But how could Paul say this, when some were not being faithful?
First, Paul could say it because they were still showing good life-signs in important areas (cf. vv. 3-4). Second, he could say it because of tact. Although Paul is intending to correct them quite sharply, he shows wisdom (cf. Proverbs 15:2a──"The tongue of the wise makes knowledge acceptable" [NASB]).
Paul is not saying that every individual Colossian church attender was flawlessly faithful. Rather, he is addressing them with tact and charity, giving them the benefit of a doubt, speaking with them in accord with the best view he can take of them: personally faithful individuals who are being troubled by false teaching. Until driven to conclude otherwise, it is wise to try to hold the best possible view of a person, giving him the most credit we can, giving him the benefit of a doubt.
In other words, Paul is calling them by what they have been, what they should be — and what they still can be, by repentance and doing the word of God!
Finally, Paul calls them "brothers." Again, think of it. This is a Jew calling Goys "brothers." In Paul's day, one common nick-name by Jews for Gentiles is found in Ephesians 2:11, where "uncircumcision" is literally foreskin. Crude, and far from nice. In fact, it was said that God created the Gentiles simply as fuel for Hell.
So, we shouldn't gloss too fast over this pure-bred pedal-to-the-metal Jew calling this church-full of Gentiles "brothers"; how can he? He can do it because they are "brothers in Christ." (Read 1:26-27, then read 3:11.) Although the Jews still have a distinct future in the plan of God, and still have a covenant with God, in Christ we are all on equal footing: all equally redeemed, forgiven, regenerate, and loved by God.