18greens - that's all I remember too!
I learnt them just enough to pass my PStar exam then promptly forgot them
I now remember I was visiting a tower somewhere a while ago. They had one of these signal lamps (with quite a layer of dust on them, I might add). And they had a nice little crib sheet attached to the lamp that listed all the light signals.
So it's not just pilots who promptly forget the signals after their exams. It's the ATCOs too.