CLARITY

Paul's final reference to teaching in Colossians is found in chapter 4 where he asks the congregation to pray for him as he "speaks".

His prayer request is twofold: First, that God would present opportunities, and second, that when those doors opened, Paul would be prepared to make "the mystery of Christ" "clear": 

2 Continue steadfastly in prayer, being watchful in it with thanksgiving. 3 At the same time, pray also for us, that God may open to us a door for the word, to declare the mystery of Christ, on account of which I am in prison— 4 that I may make it clear, which is how I ought to speak.

So it seems the final lesson on effective teaching in this letter is that it does not just happen by opening one's mouth. 

The goal of effective teaching is clarity, and prayer is the facilitator.