[Theseus]

The paint roller thingy would be most sensitive to paint. Let's say there was Yellow (or Gold-colored) paint on a wall somewhere. Then, quite naturally the paint-roller-handle thingy would point to the subject wall.

Also, it matters not if you label what is being done as "mental" or "physical". The signal in question is neither AF, RF, ELF, UHF or VHF. In fact, the signal that causes the paint roller thingy to respond comes from the operator's mind, is transmitted down their arm, and causes their hand to tilt and this makes the paint roller thingy move about. All really simple, wouldn't you say?