Even prototype colors vary in the wild due to sun bleaching and dust.
I've found that MTH sometimes uses a different color -- i.e., the blue on a Santa Fe diesel might be the blue CSX uses. Taking the shell to a well-stocked hobby shop is probably the best approach. I'd only use an official railroad color for something undecorated or for a full re-paint. As for freight cars, use the "official" color for the paint as it would represent a new paint job, then maybe hit it with some dust for slight weathering.
