Yup the truck engines were mostly different colors.
IIRC the early 40s-60s gray-blue and blue is not the same as the late-70s GM Corporate Blue
Funny, back in the Model T era most all engines were black.
Then everybody went to dark green of some flavor.
Then came the multicolor era of the 40s-60s.
Then everything went blue for a while (GM, Ford, Mopar, and AMC)
Then it all turned to gray and then back to black like the Model T era again. You cant see the engine in the new ones anyway so I guess it doesn't matter.
I try to keep a chart of all the engine colors I can, the link Lifeguard posted seems accurate AFAIK with the right caveats.
I got so tired of everything being orange that I did my 292 in Diesel Green for the Nova with a truck "292 Hi Torque" decal just for fun. Well, it IS Hi Torque compared to the 194 that came in there!
