here's a quick method.
Set up a generic dynamics particle spray directed at the wall (a spray can)
Make the wall collidable for that particle system.
Under dynamics, select collision events - make a new particle be emitted whenever a particle from the 'spray can' hits the wall, and kill the old particle when it does so. Make sure that no energy is conserved, so the particle won't go anywhere (or very low). Watch out if you decide to have gravity affect it though.
If you're only doing one color, simply create a new shader for the particle system, and you're done.
If you're trying to get a whole painting, you might be able to map it to the perParticle attribute, based off a file. But I'm not sure about that.