Colossae was about 100miles East of Ephesus, and one of three cities in the Lycus valley. The other cities being Laodicea and Hierapolis. Although Colossae had been a large town by the time of the NT it was small with a cosmopolitan population This meant that it had many cultures and religions there.
We note that Paul instructed the Colossian Christians to send the letter to Laodiceans so they could read it and they to read the Laodicean letter Col.4:16. The error seemed to have been among the Laodiceans as well Col.2:1.
The letter to the Laodiceans is lost to us -- unless it was a copy of the Ephesians letter. Academics have suggested that, from internal evidence, the Ephesian letter was in fact a circular letter to many churches. If that be so then we only have the Ephesian copy left to us.
What we do know is that Ephesians was written about the same time as Colossians, Philippians and the personal letter to Philemon. All of which were written during Paul's first Roman imprisonment. That being so we can see, although the emphasis in each letter is different, there are many common expressions and thoughts in them. This will be be helpful in our understanding in our studies