Here's a novel idea: an ink pen that reads the colour of any object you place in front of it, then mixes red, blue and green ink to match it.
But I'm skeptical as to whether or not you can fit all the parts and electronics needed into a usable pen. You'd need room for a power source, refillable ink cartridges and a chamber for mixing ink. Still, the idea is pretty neat, is it not? Marker companies would become obsolete. [I new Idea via Oh Gizmo]