This is where I depart from the purists and the color police - I don't mix colors unless it is for a very small area or part, that I can paint all at once or at the same time, because it seems I can never, ever match it again if a do-over is needed. It's either out of a bottle or it doesn't get done.
Close enough is close enough 99 percent of the time - do you really want to agonize over which minutely-different shade of gray is the correct one for your ship? I don't, at least not anymore. I research the color chips, go to my World's Largest Collection of Gray, Green and Brown Paints, and pick the one that's as close to it as I can get.