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Mac, my hunt for CIM equipment has come up empty with anything in that blue color. Most of their freight cars were either yellow or light green, like their engines. A lot were also typical boxcar red. I'd used a CSX blue mixed with some white and call it good. I don't think anyone else its going to be able to challenge you on this one.
I'm building two of these for a customer. He gave me the 2 pics I posted, the box cars and old Champ decals. The decals call for green but which green. Was the green close to pullman green or a special mix.
I think what I will do if I can not find the info by Friday I will paint a test sheet of plastic and have him pick the color from 3 or 4 greens that I have on hand.
Sure doesn't look anything like that boxcar to me. I guess painting up some samples and showing them to your customer is the best bet since he presumably saw these cars in person and can tell you if you're close.