Thanks for replying.
In answer to your question:
Why not make a stencil the exact size and shape you want?
The robot will be printing binary images sent to it on the fly.
You might even be able to get away with using paint from a spray can, if the robot is able to press and depress the top of the can quickly enough (something that a solenoid with a return spring could potentially do).
This could potentially work, except that we need to use a specific type of ink that is easily erasable. Maybe there's a way to get that kind of ink/paint pre-packaged as an aerosol.