The gold card is for suite guests only. As with other C&A members, our member level is printed on the standard white card. I assume if a diamond member sails in a suite, they'll have a gold card that also says Diamond Member on it. Diamond and Diamond Plus members do get key cards for the concierge lounge.
Personally I don't think the color of the card or the level of membership printed on it makes a big difference to the staff in most cases. It is mostly eye candy for the suite guest.