The obvious type of fair randomness is a symmetrical distribution (equally likely to give N more blue than green, as to give N more green than blue), and this gives equal chances of blue or green to come out of the bag. If I knew it was a double blind experiment, so the experimenter doesn't know the contents of the bag, I would treat red, blue and green as each having known probability of one third. If the offers might depend on the experimenter's knowledge of the bag contents I would not.