The comprehensive rules state that
You must ensure that your face-down spells and permanents can easily be differentiated from each other. You’re not allowed to mix up the cards that represent them on the battlefield in order to confuse other players. The order they entered the battlefield should remain clear. Common methods for doing this include using markers or dice, or simply placing them in order on the battlefield
If my opponent makes an assumption about a facedown card based on public information that is wrong, am I required to correct his assumption?
For example, suppose I'm using a red, green, and blue markers to differentiate my morph creatures. I have a blue marked, red marked, and green marked creature on the field. I turn the red marked creature face up, triggering Deathmist Raptor's ability. I choose to return it to the battlefield face down, and put the red marker on it. Since it came from a public zone, my opponent knows that Deathmist Raptor is the red marked card.
Later, I attack with all three morph creatures and my opponent is thinking out loud when blocking. If he says something like "Deathmist Raptor is the blue marked creature", am I obligated to correct him? Alternatively, is he allowed to ask, "when Deathmist Raptor entered the battlefield face down, what color marker did you put on it?"