Painting with sound, awesome! The artist Martin Klimas splattered paint in different colors over the diaphragm of a speaker. Then turning the volume up, the vibration of the speaker sent the paint aloft in patterns.
Use washable fingerpaints and paint-friendly clothing (ahem). Pair up child with parent each "armed" with paint and let them dab fingers in colors and smear as both giggle and go to WAR! A fun pairing game for this kind of party would be Twister.