The thick plottens... It seems likely that Marietta BSed about her investigation and Teresse's roses were the only ones to change color. As to what that signifies, SHE seems to know and, uh, appears to think it important enough to do a disappearing act with Teresse. What she thinks she'll gain by that I don't know, all she's doing is making herself look mighty suspicious.
(She can't really claim the one red rose is hers without being immediately called a liar, multiple people saw Teresse pick it up.)