Okay, so there’s the day when Regina reveals to Kathryn that David is having an affair (Regina is wearing a a black turtleneck and grey blazer). Then the next day, Kathryn visits Regina again: “I thought after what happened yesterday I’d be the last person you’d want to see.” and tells her that she’s leaving (Regina is wearing a grey top, blazer and scarf). She’s then wearing this same outfit that evening as she burns the letter and we discover Kathryn is missing.
In the scene in Stable Boy, she’s wearing a grey top and red blazer.
So I’m down to before she knew Kathryn was leaving, costuming screw-up or Regina likes to randomly change her outfits back and forth throughout the day. (Yes, I have probably given this too much thought)