I understand that players have their favourite colours, but I think it would be a good idea to prevent players from using the same colours as others because it can be confusing.
Maybe a few more player colours would not hurt either.
A few more player colors would require a rewrite of this subsystem ...
The lockout idea is a good one, I think. I am not sure that more player colours would justify heavy engineering at this juncture.
Next version shows an X for used colors.
Splendid!