Bob's hand is, from newest to oldest: yellow 3 (globally known), red 2, blue 4, purple 5.
What should Alice do this turn?
Alice should play her green 3.
Since Bob can play his yellow 3, the purple 5 is not in immediate danger of being discarded.
It would be illegal for Alice to perform a Scream Discard, because that is only supposed to be used as a last resort, and since Bob is expected to play the yellow 3 on this turn, it would not be a last resort.