I'm x-posting this from a talk page on the RS3 wiki, because it's relevant here too (and I might get a reply!).
I am positive that the strength potion was originally purple, but this was changed in a hidden update some time during 2001. I had a source for this several years ago (an old archived item database, I believe), but don't have the URL any more. I'd be very happy if someone could find the source and add this to the trivia section. If not, I'll try searching a bit more.18:05, November 26, 2014 (UTC)