If the speaker is painted with "krinkle" or "wrinkle" paint, this can often be found at your hardware store. If your hardware store doesn't carry any, you can ask them to order some in the color you desire. www.tubesandmore.com sells this paint in several shades, too. Also, if you can't find the shade you desire, sometimes you can use a different shade and then put a coat of regular paint in the shade you desire over the krinkle paint. If this is done carefully, it'll turn out fine. Unfortunately if you scratch the surface layer the color beneath will be exposed.
Thomas