I noticed a couple things about this episode, and when I re-watched it recently, I found a couple more.
To some degree, this is also in the previous episode (and perhaps even in some before that), but it's very noticeable in this episode that Juliette is wearing a coat that is much darker than ones she usually wears:
The darker color seems to represent the darkness involved in her accepting and even reveling in being a Hexenbiest and in her joining forces with the royal family. She's become a literal turn-coat.
She takes Prince Kenneth on a tour through the house she used to share with Nick, and when they get to the bedroom, she reaches out and touches the dresser drawer in which Nick keeps the ring with which he proposed to her:
There are a few other touches like this in the episode (like the slight hesitation before she tells Kelly to come in) that seem to indicate the old Juliette within her new Hexenbiest-self.
Meanwhile, Renard is being taken over by Jack the Ripper. In one scene, he looks at himself in the mirror, which seems to spur Jack's taking over:
I think using mirrors to show a dual nature is a relatively common feature, but it works well here. Like the original and the reflection, there's Renard himself, and then there's Renard as he is when Jack the Ripper takes over. Using a mirror to illustrate this is particularly evident when - in a later scene - Jack the Ripper writes on the mirror with the blood from Renard's phantom wounds:
Finally - and this is just a minor point - when Nick and Hank find that Wu has been taken by Renard/Jack the Ripper, Nick calls dispatch and says, "I need to track patrol car 421." 421 is the number of the car, and this is season 4 episode 21. Using the season and episode number in the episode itself is a common conceit in Grimm, but usually it's done in an apartment or hotel room number. Here, there's not even a visual component.
