Paul didn't serve alone. A team of six exemplary men surrounded him. These brothers, like Paul, cared for the Colossians and sent their warm greetings.
Jewish brothers (10-11): The first set of greetings was from Paul's Jewish companions ("from the circumcision"). Aristarchus was the first man, whom Paul calls "my fellow prisoner" since he, too, had been imprisoned for Christ.[1] The second man was Mark, Barnabas' cousin. Paul restored him from his past failure and reinforced instructions that he be well received.[2] The third man was Justus who had the common Hebrew name, "Yeshua." These were the only fellow Jews who labored in the kingdom work and gave him comfort.[3]
Gentile brothers (12-14): Greetings also came from three Gentiles. First was Epaphras, a native of Colossae, a "slave of Jesus Christ,"[4] who prayed "always laboring earnestly" for their maturity ("perfect") and for their full conviction in all the will of God.[5] This man had a big heart and prayed this way for the saints in Laodicea and Hierapolis also. Finally, Luke and Demas, Paul's fellow workers,[6] sent greetings. Luke authored Luke and Acts, and he was with Paul to the very end (2 Tim. 4:10-11). Ministry advanced through this teamwork.
[1] "My fellow prisoner" does not necessitate concurrent time and place (cf. Rom. 16:7). Aristarchus may or may not have been imprisoned with Paul in his many pre-Roman imprisonments (cf. 2 Cor. 6:5; 11:23).
[2] This is John Mark (Acts 12:12), who deserted Paul and Barnabas during their first missionary journey (Acts 13:5, 13). At first Paul did not restore him to the work (Acts 15:37-38) but eventually did, as seen here. Paul speaks of the "instructions" in a roundabout way suggesting someone else gave it and Paul here reinforces it.
[3] "Encourage [pa-rē-go-ri-a (παρηγορία)]" only occurs here in the NT. It's best seen as "comfort" (ESV, NKJV).
[4] Paul applies this title to himself (Rom. 1:1, et. al.) and to Epaphras and to others (1 Cor. 7:22; Rev. 1:1).
[5] God wants us fully convinced in all His revealed plan/purpose (cf. 1:18-20, 27; 2:2-3; 3:3-4, 15-17).
[6] Cf. Phm. 23-24. Demas later deserted Paul because he loved this world (2 Tim. 4:10). The absence of a commendation does not have to mean Paul had a hunch. He listed others this way with no ill (2 Tim. 4:21).