The original toy prototype (as shown in catalog images http://tfwiki.net/wiki/File:G1catalog86-PinkHotRod.jpg ) was also magenta, so that's likely what was intended before the change, but the movie was probably already in the can by then.
The Masterpiece Hot Rod/Rodimus Prime figure released in Japan a few years ago was also magenta-ish, but the Hard Hero bust was a closer match to the colors: http://tfwiki.net/wiki/File:HardHero_HotRod_bust.JPG