Hi BS,
You are correct that the reels were originally more of an "Orangey" colour opposed to Red but most people prefer the red and thus why it's used more I guess. The orange just looks a tad dated on the reel, while the red looks a tad sharper today.
Yes you are correct regarding the engine manufacturers using their own colours and supplying a finished product to each individual product manufacturer to simply fit straight on.
Briggs used a somewhat different colour to Kirby. The Kirby shade is a tad deeper with a bit more of a red under tone while Briggs used a brighter orange similar to Chevy Orange.
Cheers,
BB.