Note 4 at Col. 4:3: "Withal" is a word meaning "in addition or besides" (AH Dict.). So, Paul is requesting prayer that God would open "a door of utterance" in order that he might preach the gospel.

It is amazing that Paul is requesting more opportunity for speaking the gospel. He's in prison for that very thing at the time of this writing. Yet Paul is undeterred. His whole life centers around doing God's will regardless of the consequences.

This is similar to the apostles prayer after having just been imprisoned. They prayed, "And now, Lord, behold their threatenings; and GRANT UNTO THY SERVANTS, THAT WITH ALL BOLDNESS THEY MAY SPEAK THY WORD, By stretching forth thine hand to heal; and that signs and wonders may be done by the name of thy holy child Jesus. And when they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and THEY SPAKE THE WORD OF GOD WITH BOLDNESS" (Acts 4:29-31).