Matte is an additive to paint. If you are "rattle can" painting, you can get flat black and possibly other flat colors as well. If you are using a compressor/gun/auto paint, then you add flattening agent to the final clear coat(s). As a note (from my personal experience) if you add too much flattening agent, then the clear becomes like fog and will add a white tint to your color. BIG BUMMER! Repainting will be needed to fix that glitch.
Hope that helps,
Dr. D mb at mbent dot net if you have more Q's.