My son came home from school the other day and asked me if I could get pen off his favorite hoodie. Another student had taken a few colored Sharpies and drew lines down his back. I told him I would see what I could do.
I poured isopropyl alcohol directly on the spots and used Q-tips to scrub across the pen marks, rinsing it out and reapplying a few times. After a few minutes, the marks were lightened to barely visible. You can see where the alcohol and scrubbing lightened the green sweatshirt color a bit. I didn't want to damage the material any more so I stopped.
This was just a cotton sweatshirt. I would use caution on any delicate fabrics and test a small area first.