"The book of Romans is the flagship of the Pauline fleet. Flying proudly at the top of the mast of this ship is a flag bearing the words, 'Paul, a slave of Christ Jesus' (Rom 1:1). This flag is two-toned, its white indicating complete freedom yet total surrender, and its purple symbolizing royal ownership and therefore incomparable privilege. The slave of Christ is the emancipated dependent of Christ as well as the willing bondservant of Christ, the exclusive property of Christ as well as the honored representative of Christ."
Murray J. Harris, Slave of Christ: A New Testament Metaphor for Total Devotion to Christ, page 155.