Critical cards are defined as the final card of a suit and rank combination that is not discarded yet. For example:
If Alice has a red 2 in her hand, and the other copy of red 2 is in the deck, then Alice's red 2 is not critical.
If Alice has a red 2 in her hand, and the other copy of red 2 is already discarded, then Alice's red 2 is critical.
If Alice has a red 5 in her hand, then it is always critical. (There is only one copy of each 5 in a Hanabi deck.)
Obviously, critical cards need to be given Save Clues so that they are not discarded. (If a critical card was discarded, then the maximum score of the game would decrease.)
As long as the critical card is not a 5, we agree that players can save critical cards with either a color clue or a number clue.
In the example below:
Alice clues Bob blue, which touches a card on slot 4.
Before this clue, the only clued card in Bob's hand was the 5 on slot 5.
Bob's first instinct is that this could be a Play Clue on the blue 2. However, since the clue was focused on his chop, this could also be a Save Clue.
Bob looks through the discard pile and finds that there is a blue 4 there. Thus, this could be a Save Clue on the blue 4. So, Bob knows that for now, he has to hold on to the card.
Bob marks the card as either a blue 2 or a blue 4, so that he can keep track of it for later. Then, he discards.