Even if we are under grace, aren't we supposed to do certain things in obedience that show God we are serious about Him (and show others we are serious about God)? Bringing in mixture to do certain things adds sin consciousness. This takes our focus off of Jesus and onto ourselves. We judge ourselves to see if we are pleasing to God or if we are fruitful. These things are good but in the book of Colossians, Paul is telling us how to do this...by pointing to Jesus.