The young guy who did this would be very wise to lay low and not reveal himself online. He could end up with a criminal record that will follow him around for the rest of his life. Every job application asks for your criminal record.
So, just a warning to other foamers to think twice before they pull a prank like this. Don't ruin your life.
The repaint back to yellow paint won't hurt the UP's bottom line or stock value. But they will still go after the prankster with a vengeance, only because corporations are heartless and so are most people.
Have fun, but stay out of trouble.