@joedix maybe it would be worth asking the restaurant next time you eat there?
Yeah, I have thought about asking but I realised that I have too many questions to bomb the lady that's serving :D
As I said, there are two kinds of bok choy. They are both dark green. They will remain dark green even overcooked. If you Google Image you you will see what I mean.
Napa is yellow, and some times very light green. Napa is used often because they taste good and about 1/3 the price, and they can be kept for along time without wilting.
Overcooked bok choy