Well, the guess as to it's original colour would be about as accurate as guessing what colour a house built in the 1840's originally was, there's three colours I can see on it, Classic Omega Brown, ClassicIII Red and Sanitronic Tan, the most likely would be the Tan colour as the handle lock and footswitch are tan (though the footswitch button can be pulled out and swapped easily, the handle lock trim is fiddly to remove), so, unless it was completely rebuilt with Tan parts in the past, I would give Tan as an educated guess... :\
That's the problem with vacuums that have been rebuilt in this manner, they lose a lot of their originality so that it's pretty much a beast of many parts, hence "Frankenkirby", once cleaned up and refurbished, it'll work fine and do everything it's meant to, but it's a bit of a mutt and would take more money than it's worth to get it back to original, but at the end of the day, if you're happy with it, then that's fine, people are happy with rebuilt models with many colours, and others are only happy with all-originals, it just depends on the owner...
At least your Kirby has an ID plate, my D80 hasn't as it had been removed in the past and not refitted, so I know it's 1967-70, but I don't know when specifically it was made, but I love it anyway...
