Board Game Arena

I'm noticing a lot of Hanabi players immediatly play a card when you clue them a color or number. But what if a number is in danger because it's been already played? For example the three of white has been already played. Then how to clue that it shouldn't be discarded?

First of all: don't clue it until it is actually in danger of being discarded. Don't clue if it's not the rightmost unclued card (a.k.a. chop). Don't clue it if the player has a clued (or finessed/bluffed) card to play. You don't need to clue it (but you can) if you can force 8 tokens to the player, so they cannot discard.

Typically, at least on BGA, a number clue touching the chop is considered a save clue and not a play clue by default. Why number and not color? Because the first cards we need to save are 5s, which can be clearly identified with a number and not a color. Also a number clue is always possible even with black powder.