Psalm 89:27 declares that God will make the Davidic king His "firstborn," the highest of the kings of the earth. In biblical usage, "firstborn" can denote rank, privilege, and preeminence rather than simply chronological birth. The parallel phrase about being highest among kings makes that sense especially clear here.
Colossians 1:15-18 calls Christ the firstborn of all creation and the firstborn from the dead, immediately explaining His supremacy as Creator, Sustainer, and Head of the church. Paul is not teaching that Christ was the first creature; he emphasizes Christ's preeminence over creation and the new creation. Although Colossians does not quote Psalm 89:27, the royal concept of the firstborn as supreme finds a fitting culmination in Jesus, the Davidic Messiah who is preeminent in everything.