Though rare, it is possible to have a no valid clues to give on the first turn. (Specifically, no playable cards, no 2's on chop, and 5's to 5 Save or 5 Stall with.) Thus, since any clue will cause a misplay to occur, we agree to give the clue that causes the least amount of damage (or the most benefit).
The best clue will depend on the particular game.
For example, you might want to cause a misplay of a card that will not be a bottom-deck risk:
Alice sees no valid clues to give on the first turn.
Bob has a yellow 4 on chop and Cathy has a yellow 4 in slot 2.
Alice clues Bob yellow, touching the yellow 4 and representing a Play Clue on a yellow 1.
Bob misplays the yellow 4.
Cathy assumes Alice had no legal clues to give on the first turn.
For example, you might want to lock-in a save one or more cards:
Alice sees no valid clues to give on the first turn.
Bob has a a purple 4 on his Finesse Position.
Cathy's hand is, from newest to oldest: x, x, red 3, blue 3, yellow 3
Alice clues number 3 to Cathy, touching three 3s and representing a 3 Bluff to Bob.
Bob misplays the purple 4.
Cathy assumes Alice had no legal clues to give on the first turn.
Normally, when a multi-color card is Prompted from someone's hand, they should play the card that has the most positive information.
However, what if they are Prompted for two cards of the same multi-color suit? They should play the card with the most positive information first, even if it is not the left-most card.
For example, in a 3-player rainbow game:
Rainbow 1 is played on the stacks.
Bob has a slot 1 card that has a blue clue on it. (Thus, it is either a blue card or a rainbow card.)
Bob also has a slot 3 card that has a blue clue and a red clue on it. (Thus, it is a known rainbow card.)
Alice clues Cathy about a rainbow 4.
Bob is promised the rainbow 2 and the rainbow 3, so he knows that these cards must be on slots 1 and slot 3.
However, Bob also knows that you should always play the card with the most information on it first, so he plays his slot 3 card as rainbow 2 (instead of slot 1, the left-most card).
Normally, when a playable card is discarded, a player will write Elimination Notes on their hand for the other copy of the card.
However, there is a special case when two copies of the same 1 are discarded. The original Elimination Notes from the discard of the first 1 no longer apply because it is possible that the player has drawn the third copy of the 1 before the second copy could be clued.
Thus, when a player discards the second copy of a playable 1, they should delete all of their Elimination Notes from the first discard and rewrite all of the Elimination Notes on the rest of their hand anew (essentially acting like the second discard was the first discard).
For example, in a 3-player game:
Nothing is played on the stacks.
Alice discards a red 1. Alice writes Elimination Notes on her slot 2, slot 3, slot 4, and slot 5.
On her next turn, Alice discards an unrelated card. She now has Elimination Notes on slot 3, slot 4, and slot 5.
On her next turn, Alice discards another red 1. Normally, she would now have Elimination Notes on slot 4 and slot 5. However, because of the Elimination Rewrite, she instead writes Elimination Notes on slot 2, slot 3, slot 4, and slot 5.
Given enough negative clues on a card (and potentially some ancillary information based on the history of the game), it is possible to narrow down the identity of an unclued card in your hand to possibilities that only include playable cards that are not already touched in someone else's hand.
In this situation, you are expected to blind-play the card.