ὅς ἐστιν εἰκὼν τοῦ θεοῦ τοῦ ἀοράτου, πρωτότοκος πάσης κτίσεως (Colossians 1:15 W-H)
καὶ αὐτός ἐστιν ἡ κεφαλὴ τοῦ σώματος, τῆς ἐκκλησίας· ὅς ἐστιν ἡ ἀρχή, πρωτότοκος ἐκ τῶν νεκρῶν, ἵνα γένηται ἐν πᾶσιν αὐτὸς πρωτεύων (Colossians 1:18 W-H)
Some want to make much of the fact that the Greek preposition ἐκ appears in 1:18, but not in 1:15.
ἐκ in 1:18 could be used in the passage to emphasize Jesus' resurrection from the dead, as one friend of mine has suggested; but there is another explanation that may also account for ἐκ without resorting to a Trinitarian alternative.
Petr Pokorny (Colossians. Peabody, MA: Hendrickson, 1991. Page 84) writes in ftn. 153 concerning 1:18:
"MSS P46, Aleph (first hand), and others omit ἐκ = from. The sentence reads the same way in Rev 1:5. The meaning is not altered thereby."
Revelation 1:5 has ὁ πρωτότοκος τῶν νεκρῶν. It evidently means the same thing that Colossians' πρωτότοκος ἐκ τῶν νεκρῶν does, as the MSS evidence indicates. It just seems that ἐκ is normally used when the resurrection of Jesus Christ is under consideration. See John 21:14; Rom 4:24; 6:4; 10:7; Col 2:12; Gal 1:1; 1 Pet 1:3, 21.
Meyer's NT Commentary: "comp. Revelation 1:5, where the partitive genitive τῶν νεκρ. (not ἐκ. τ. ν.) yields a form of conceiving the matter not materially different."