I know many players use a line on their ball to show the starting line for a putt. I believe this violates the Rules of Golf for the following two reasons:
1. I believe having a line on the ball violates Rule 8-2b. The line on the ball is present when the stroke is taken and, by definition, it's a mark.
From the Rules of Golf:
"8-2. Indicating Line of Play
b. On the Putting Green
When the player's ball is on the putting green, the line of putt may be indicated before, but not during, the stroke by the player, his partner or either of their caddies; in doing so the puttin