We label our source fours with spike tape around the yoke and there isn't a problem with them coming off. Its practice to not swap barrels but instead just swap fixtures. Ive worked places where they paint the gel clip area and then use colored magnetic tape on the yoke. The fixtures dont get beat up too bad so the magnets stay on.
I prefer to have both the lens tube and the yoke labeled because depending on how you look at it one location is easier to see than the other.