To get paint pen off your car you can apply some rubbing alcohol to a clean cloth and then gently dab or wipe the paint pen marks off the car.
The rubbing alcohol can help to dissolve the paint pen ink from the car surface.
You can also use some mineral spirits or hairspray or even WD-40 although the mineral spirits and rubbing alcohol will work the best.
Even using some acetone based nail polish remover can help to remove the paint pen and other inks off your car.
If the paint pen ink is stubborn and not coming off the first try you may need to try it a few more times to get the paint pen ink off the car.
I've had good luck using rubbing alcohol and a clean cloth to remove paint pen ink off of my car.
When applying the rubbing alcohol or other solvent to the paint pen ink, start by dabbing the area with a cloth and allow the rubbing alcohol or other solvent to sit on the paint pen ink a few minutes before wiping it off as it allows time for the rubbing alcohol or other solvent to dissolve the ink before you wipe it away.
Repeat the procedure as necessary until the paint pen ink has been removed.
If the paint pen ink is being really stubborn you can also use a light abrasive such as baking soda or toothpaste with the solvent to help remove the paint pen ink from the car.