If I were doing this, I would run two seps. First an automated grayscale separation which would pull multiple grays, base, highlight, black. It would be a pure grayscale separation of the entire graphic since there's a ton of subtle grays in there.
Then work on a duplicate and pull the colors. Once satisfied with the other colors, I'd shift-drag those channels into the grayscale sep then knock them out of the gray channels. You could play games with the hue-saturation of the image prior to running the color sep to grab enough data and then adjust the channels accordingly, change their color, etc.
BTW, I no longer do custom seps as I see this as competing with my users. Plus time has become scarce lately.
Good luck.