I am still trying to find the correct red color for GM&O.
You may have to mix it from paints close. If you just want a reference i suggest you rent the movie In the Heat of the Night with Sidney Poitier. It was on PBS here in Chicago a couple of weeks ago and used GM&O passenger trains at both ends of the movie. good movie too.
Might want to try the GM&O historical society site at www.gmohs.org. They have a modeling section that may be of some help to you.
God luck; the GM&OHS website is pretty bare. Your best bet is to ask on the IC or GM&O YahooGroup modelling lists, or to head over to the Fallen Flags website and stare at color pics of GM&O diesels.
Rember, lots of things affect scale color. In the case of the GM&O, it’s dirt and exhaust, combined with fading and a general lack of washing and maintenance (Go-Mo diesels were ALWAYS filthy, and looked like they were falling apart, even when they were relatively new). Just pick out to colors that look “right” and go from there. To me, the red looks like SP Scarlet and the maroon looks like Soo or Milwaukee burgundy. They might look different to you, but we’d both be “right”.