Oh, I don't mind admitting that I'm wrong WHEN I'M WRONG.

Assuming only these four cards you just might could turn card A over and find an even number. You haven't proven the hypothesis false. If you then turn over card 7 and find a vowel, then and only then have you proven the hypothesis false. And in this case turning over just the 7 card will be sufficient to prove it false and you can take your A card and paint it green for all I care.