Nicely done.   I would also suggest of course that the range of characters used in the codes omit certain characters such as “Oh” and “El” and “Ess” so that substitutes zero, one, and five (respectively) can be automagically substituted.

I would further suggest that the text be as large as you can make it, and that the difference between acceptable and unacceptable responses be more than “red and green.”   My dad, and a couple of my aunts(!), could not tell the difference between the two colors.   (They drove into one town where the traffic lights were inexplicably mounted upside-down and almost had a wreck.   The same thing happened when the lights were mounted sideways.   Clues such as texture, or a background difference such as a red “X” vs. a green “check-mark,” are also helpful.