Everything about the photo says to me that it's both a poor scan and a badly aged slide or negative. This slide is of a similar vintage and there's a few things you can compare between the two:
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The sky in this slide is a light blue. Obviously it's just a picture of the slide through a slide viewer, but it still gives a reasonable impression of the color. The overall color cast of this slide leans towards that more blue tone, while the 'green' SD40-2 has a strong pink tint. RRPictureArchives is not loading for me right now, so I can't see any of the image details, but this sort of color shift is somewhat common with older slide films. Kodachrome is extremely stable in its colors, while pretty much everything else starts to break down after about 40-50 years after processing. Ektachrome in particular tends to get the same purple/pink shift that you can see in that picture. My guess as to what happened is that someone scanned a deteriorated slide, which had also lost a lot of its contrast, and the scanner overexposed the image as a result. In doing so, it tried to bring the black of the locomotive up in brightness, but there wasn't enough detail left in the scan to actually do this, so you get an image that's basically just digital noise. There's also probably some environment reflection going on here; it looks like a clear day, and a clean locomotive will pick up a lot of that light and scatter it. It's an extreme example because of how polished it is, but consider the photo below:

Simply using a color picker and sampling the image will tell us that it's supposedly painted #495b81, a deep blue which coincidentally happens to be quite close to the color of the sky. In fact, going back to the first slide in the post, sampling the body appears to show the color as #273248, an even darker blue, despite the fact that it appears to be black to the eye. Now of course, because the boiler is round, there's a part of it that won't be reflecting the sky, and this tells us the real color is a deep black. On something like a diesel, though, where the sides are (mostly) flat, you often won't have this luxury when looking at images. Black is particularly bad because it will pick up just about everything in the environment, from the sky to trees to buildings behind the photographer so long as it's kept relatively clean. I would be extremely cautious of picking colors directly from photos, and even moreso about picking colors from film photos. I would really only trust digital photos for color sampling as they do not deteriorate with age, and are not susceptible to errors in scanning (which itself is more of an art than an exact science). Even then, I'd only sample for a base, then continually try colors until you get something that looks to your eye to be an average of a couple of photos of the thing.
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Matt J.