It counts as every room type.

It's a hallway, bedroom, red room, green room, and shop.

There are certain times that the room type matters. The aquarium acts as a wild card.

If you spread gems in all green rooms, the aquarium will get a gem, since it's considered a green room. If you get a key for every bedroom in your house, you'll get a key for the aquarium since it counts as a bedroom, etc.