In some physical versions of hanabi, only 5 rainbow cards are available, one of each rank. For example, in this version of the game
http://www.cocktailgames.com/en/game/hanabi/
on the back of the box it says (translated from French) "contents: 55 [cards] (of which 5 rainbow cards)."
Also, my friends have a Dutch copy with only 55 cards total. In their rule book, there are variants "6th suit, one of each rank" and "rainbow, one of each rank".

So arguably, "rainbow, one of each rank" is a (semi-)official variant, and it's always fun to speedrun in more variants. So I hereby request to add (for each player number) the variant "rainbow, one of each rank".