The Magic Paint Spell allowed Alexander to paint a door onto the side of the Castle of the Crown and gain secret entry to the basement. The Magic Paint Spell is found in the Spell Book that Alexander received in trade from Ali's Bookstore.
The ingredients are as follows.
Add Styx Water to Swamp Ooze in Cup. Stir with Horse Feather. Apply to blank surface with artist's brush. Speak incantation over painting to materialize.
Magic Paint, black
Bring to life, what
Make it real, what
According to this