After the GM&O's merger with the Chicago and Alton, GM&O passenger trains and freight locomotives were painted a maroon color with a red stripe, edged in yellow. Here is a picture from another website: http://www.trainweb.org/jimrail/b70.jpg Here is a picture of an N scale model that might be more helpful: http://ntrak-bn.adventurelandhobbies.com/pages/gm%26o%20%233.htm Personally, I think the maroon here is too red, but most pictures I've seen show some variation in the shades. There was a painting chart published in an early issue of Mainline Modeler back in the early 1980s. In the later 1960s, up to the end in 1972 there was a tendency to simplify the paint scheme, usually some shade of red or reddish maroon with yellow lettering. You would do well to find a copy of Don Ball, Jr.'s America's Colorful Railroads, or the GM&O Color Pictorial for some better photos over the 1950-1969 period. You might also want to look into the GM&O Historical Society. As for the names of the colors, I forget what the prototype colors were called, but for some reason "Dulux Red" and "Dulux Maroon" stick in my head. I believe there is one line of model paints that markets a "GMO Red" and "GMO Maroon", but I forget which brand. Personally, I'd look at some good photos, and see which model paint chips match. (Or compare paint chips with your already painted equipment.) I hope this helps.
Dan