Hello there,
I came across this trying to help my mom figure out what's happening with her exposures now and then. She's an amateur and has her film developed at a local lab, nothing fancy. Every now and then in the past few months an image will be color shifted in a major way - a pink flower will be aqua, a red one dark purple. It's like that on the negatives already, very very clearly, not a scanning issue. It's quite mind boggling, it is individual images, not the whole film. She almost always uses consumer 200 ISO Fuji film, typically the same consumer zoom lens and practically no filters, sometimes polarizer. She's been shooting with a Canon EOS 650 for about 25 years, never seen anything like it.
I have no explanation whatsoever (I'm a wedding photographer myself, albeit digital). Thought I'd mention this here... curious to see if there will be any more thoughts on this!
Nina