simple yet hard at the same time. Re-brush it; that's what I did to my '66 when I photoshopped it. Just don't add the highlights and reflections; that's really the only difference between gloss & flat paint. It's got nothing to do with saturation, you need to brush over the reflections it has, and make it a smooth gradient of color, with subtle highlights, nothing spectacular or overly detailed.
Like I did with these two versions of our truck:
