Hi. It's not the fog of war, though that is the most common method of hiding things. If you want to hide an object from specific players, without a FOW zone, you can use: Object.setHidden("Blue","Orange","White")
This would hide the object from Blue, Orange, White, but other players would still see it.
I use scripting zones to trigger the hiding / revealing. When a card enters the hidden zone, it gets set to hidden. When it enters the reveal zone, the hidden property is removed. The zones are strategically placed to trigger when the card is moved.

Challenges with this are that gray players that join could see face up cards until they choose a colored seat. I believe you can hide objects from Gray also, but then gray would not be able to spectate. My work around was to flip all hidden cards face down when a saved game is loaded. The players can flip them face up after each player has sat in the correct color seat.
I also used Object.createButton to write "Hidden" on the card to remind the player it is hidden.

To remove the hidden property, you use the command with no parameters: Object.setHidden()
I hope that explains enough. I could give you more specifics

Thanks so much for the response. This gives me hope for whenever I get the time to work on the mod.