Usually, the importers are pretty accurate on what they do. I'm speaking about the brass importers. Lionel and MTH do what they want on paint schemes.
I double checked my AC-4,5,6,7's and the only one I put the silver on was the AC-7, which I didn't paint with a gray boiler, so I'm 'factual' on that engine. 
What I try to do is get a picture of an engine that the manufacturer has painted and copy that.
My SP friend has THE cab forward book and, looking through it, I believe he is correct as I didn't see any, although most of the pictures are black and white, reference nor saw any that looked like they had a Sacramento gray boiler. For some reason, I love cab forwards so I like modeling them (probably because they are so different).
I like NP also and have the northerns painted up with gray boilers, including the Z-5 and Z-8. I found no evidence of NP ever having a Z-8 in Gray but I do. I also painted one of my GN Z-6's in glacier park green: highly unlike that they ever did this, but, in my GN empire, they exist!
