We usually charge an hours worth of labor, unless there is a lot of speakers to paint or multiple colors. We use a HVLP spray gun, wipe down the speakers with acetone, and use the customer's paint (make them bring us a small jar), and we NEVER let the painters do it.

We had a designer insist that the customer use a "professional" painter to paint six pairs of speakers (who said they painted inwall speakers all the time). We told the client we would do it better and cheaper but they gave in to the designer, so we made them sign a waiver saying if something was wrong they would have to pay to replace the grills and add labor for us to paint them right. Two days later I go to install the speakers and guess what, Mr. professional painter painted the grills with the scrims in. All the holes were plugged and you couldn't remove the scrims. So we ordered six pair of new grills and scrims, did it ourselves, and got to do the "I told you so" thing with the client.