I haven't seen this specifically posted, but if it has been, my bad.
Edit: For those who don't know, if you do a gesture of the correct disposition (blue/friendly, yellow/neutral, red/hostile) in front of a mimic wall, they will bow to you and go away, opening the way and dropping loot. You have two tries to guess the correct disposition…if you miss on both, they attack. General guidance has been to look at revenant graves nearby to get a clue as to at least one that is wrong (they will say something like "killed for his hostile behavior", so you know that hostile is not correct), then you try the other two and alway succeed.
I've noticed a little thing when doing gestures at the mimic walls. If you look at the gestures as a spectrum going blue>yellow>red, if you do a gesture on the opposite end of what they want (i.e. if they want a red gesture, and you use a blue), their eyes light up red in anger. So when you come across one, always start off with either a red or blue gesture, and one of three things will happen:
1) you were correct, and it opens. Yay!
2) the eyes light up red, use the opposite gesture and be correct. Yay!
3) the eyes do not light up, use a yellow gesture and be correct. Yay!
I have not triggered a single mimic wall attack since I noticed this. Hope it helps for those times there are not helpful revenants nearby!