See, I don't think it was a mistake. It's too BIG a mistake; it had to be deliberate.
My wacky theory? Most of the early Decepticons were blue and or purple. They liked the name Rumble better (because he, you know, rumbles with his piledrivers; and they were only going to use one tape-person for a while, because Frenzy doesn't show up until quite a bit later), but the red looked too much like an Autobot, so they changed it.
As a result, I'm a "Rumble is blue" guy.
So, finally getting around to watching the Japanese Headmasters... on episode three now, before hitting the sack. WOW, so much better than the US story... or most of seasons two and three, even.
Nice to see Prowl was alive, and some of the other season one characters active. Saw the epic final battle between Blaster and Soundwave, etc... I could REALLY see MP figures of Blaster, Hot Rod, Sixshot, and maybe even Arcee, depending on how popular this season was in Japan...