With limited art screenprints (think Banksy and stuff like that), printers will run off 'proofs' for the artist to view before the print goes live - these might be just like the standard or are sometimes in a different colour. Usually these are marked AP (artist proof) when they are signed by the artist. They aren't really true 'proofs' these days and are often just a way to legitimately run off extra copies of a limited edition.
Alternatively, printers might run off extra copies outside of the edition - these aren't sanctioned by the artist and don't form part of the run. They are usually kept by the printer or given to mates - they are referred to as 'backdoor' prints (because they usually leave via the back door!).
With the absence of any evidence otherwise, I think you've probably just got the movie poster equivalent of a backdoor print. Backdoor art prints are worth way less than the legitimate edition prints (maybe 10%), but I'd suspect this isn't quite the case here as the posters aren't limited as such - so you've got an unofficial colour variation, which some people might find pretty cool.
If you could prove it was done for testing a different colour, then you've got something really different and cool. But I really don't think you'd be able to now sadly, although the font being different might help.
