Ephesians 6:9And, ye masters, do the same things unto them, forbearing threatening: knowing that your Master also is in heaven; neither is there respect of persons with him.
"Masters, give unto your servants that which is just and equal; knowing that ye also have a Master in heaven." (Colossians 4:1)
After instructing the servants, Paul turns to the masters and says, "do the same." In other words, just like the servant should remember that they are ultimately serving God, they are servants of God, and should be doing the will of God. They are not to seek the applause of men or do that which is right in man's eyes, but their goal should be blameless before God, realizing their testimony of the doctrine of God and the reward which comes from God (or punishment for doing wrong).
Added to that, masters are not to threaten but remember that there is One above them - God Himself - and they are accountable to Him. Then, there is a reminder that in God's eyes, all men are equal. He holds the same judgment for the servant as well as the master.