Paul names himself at the start
Colossians begins in the straightforward style of an ancient letter: Paul identifies himself as an apostle of Jesus Christ, and Timothy is with him (Colossians 1:1). That opening gives the clearest answer to the question. Paul is the author named by the letter itself.
Timothy was a close co-worker of Paul and appears beside him in several New Testament letters. His name in the greeting shows fellowship and shared ministry, not that he replaces Paul as the letter's main author. The letter continues with Paul speaking personally about his calling, suffering, prayers, and concern for the believers in Colossae (Colossians 1:23-24).
Paul, an apostle of Jesus Christ by the will of God, and Timothy our brother,
Written for believers in Colossae
Paul wrote to Christians in Colossae, a city in the region of Asia Minor, now part of modern-day Turkey. He addresses them as faithful brothers and sisters in Christ, praying that they will know God's will and live in a way that honours the Lord (Colossians 1:2; 1:9-10).
Paul had a special concern for this church because they faced teaching that could distract them from the sufficiency of Christ. Again and again, he points them back to Jesus: Christ is supreme over creation, head of the church, and the one in whom believers are made complete (Colossians 1:15-18; 2:9-10).
Who is the image of the invisible God, the firstborn of every creature: For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him:
Epaphras helped bring the message
Although Paul wrote Colossians, Epaphras seems to have played an important part in the Colossian believers first hearing the gospel. Paul calls him a faithful minister of Christ and says the Colossians learned of God's grace from him (Colossians 1:7).
Epaphras also cared deeply for the church and prayed earnestly for them. So the letter reflects a wider network of Christian friendship and service: Paul writes and teaches, Timothy stands with him, and Epaphras serves the believers in Colossae (Colossians 4:12).
A personal letter from a prisoner
The final chapter makes the letter feel especially personal. Paul mentions his bonds and adds a closing greeting in his own hand, asking the readers to remember him (Colossians 4:18). This fits with the letter's repeated references to his suffering and imprisonment for the sake of Christ (Colossians 4:3).
Tychicus and Onesimus were sent to tell the Colossian church more about Paul's circumstances and encourage them (Colossians 4:7-9). Colossians is not merely an abstract statement of doctrine. It is a real letter from Paul to real people, calling them to hold fast to Christ.
The salutation by the hand of me Paul. Remember my bonds. Grace be with you. Amen.