How, you ask?
By buying a gazillion Mr. Clean Magic Erasers. They work great to get off so many things, like blue and red pen, from walls.
How would I know this, you ask?
Hayden, my first born, and the first to desecrate our walls in the front foyer.
How do I know it was him, you ask?
The fact that he was singing "Poke, poke, poke!" and finding this surrounding the red and blue artwork on my custom painted walls tipped me off.
Unfortunately, Mr. Clean Erasers don't get out holes poked into your walls by a pen.