Does Colossians 3:2 teach that it is a sin to think about sin?

Answer:

"Therefore if you have been raised up with Christ, keep seeking the things above, where Christ is, seated at the right hand of God. Set your mind on the things above, not on the things that are on earth" (Colossians 3:1-2).

As was pointed out in Is thinking about sin a sin? there is a difference between a momentary thought about sin (i.e. temptation) and focusing on sinful ideas (i.e. lust). Paul, in Colossians 3:2, is talking about where we should have our minds focused. Temptation is going to happen, but our response should be a rejection of the temptation. "For the grace of God has appeared, bringing salvation to all men, instructing us to deny ungodliness and worldly desires and to live sensibly, righteously and godly in the present age" (Titus 2:11-12).