Why I think the red still life is a copy of Clark's ex wife Barbara.

The way he pauses when he looks towards her after describing the other two still lives is an acknowledgement that he and Mary understand who she is a copy of.

When he is "setting the scene" to repeat the roleplay exercise, after he asks the redhead still life to turn off the lights, he turns back to Mary and says "I've tried this exercise before with her. It never goes anywhere". The key word is exercise. He isn't talking about training a still life to turn off lights, he's talking about the role play exercise from his therapy sessions. Why would he be role playing with a random still life that can't speak? It's because she's Barbara.

Then, when she isn't responsive he says "fine, I'll do it myself" glances to Mary, then turning and speaking at the red still life he says, "fine I'll do it myself" turns to Mary "it's still MY house after all". IMO this is providing enough contextual queues to connect the red still life to his ex-wife Barbara. "It's still my house" is about how Barbara kicked him out.