Robot Muralist is a computer controlled XY axis printer that uses a spray paint can as its printer head.  It can paint on any surface where you can mount the XY axis controls.  There’s also a handheld version that keeps tabs on your position in 2D space and can be upgraded to the motorized version.

They also have a Hackers Club and a Community space so that users can share their tweaks to the code or pieces of artwork that are already prepped for the sprayprinter.

Lots of fun options here for activations at events on exterior walls.