If it's a flat color, like ceiling or walls, and I can get the paint from the jobsite then I do it as part of the cost of the speaker, so no extra charge. If I have to paint match and buy a quart of paint of which I'll only need a cup, and then no use for the rest, I charge materials, and if it's really custom, like a faux paint wood grain finish or chrome or something that I'll need specialty for then I charge standard hourly plus materials.

For small jobs I use the prevail disposable sprayers. For large scale jobs or certain types of grilles (ala the Sonance Z-Series Architectural) I have a professional gravity fed Wagner sprayer, that I use. For real custom jobs I have been known to use brushes, markers, airbrushes, sponges, whatever is necessary to get the job done.