Hi Chris. I made a magnetic pencil years ago for the effect where a coin placed on the back of the hand changes when tapped. It wasnt a colouring pencil though but a normal writing pencil with a rubber. I popped the rubber out and cut a small disc off. Glued a magnet into the metal bit that holds the rubber, and glued the rubber disc back on

. If you used normal pencils but ones that were different colours this may work. I got the magnet from a kids 'Magnetix' set which worked very well.
Ps. I used a disk of rubber because it had to be for the effect, this made the rubber look used. If you want the pencil to look new then cut the rubber in about half