An interesting and much more challenging variation for experienced Hanabi players is to disallow people from saying a number or color. You point to a set of cards in another player's hand and that's it. Those cards share some attribute, and all the other cards in the hand don't have that attribute, whatever it is.

Interesting! That breaks so many common reasoning rules, especially if you're playing multicolor. I can see how that would add a huge amount of challenge.

I'd be interested to see some of the reasoning based on that variant.