I think there were a few reasons why Gordon Ramsay had Melissa trade her red jacket for a blue one.
1. Previously, he saw some talent in her until she broke down this week. Was it her team, or was it her...time to find out.
2. Because one of the previous weeks, he lost two aspiring chefs, Eddie, I think, and Aaron due to medical issues, he had an additional week to mix things up.
3. It keeps these contestants on their toes when he does unexpected things, like overruling their nominations or sending her to the men's side.
Chef may be a bit of a flirt, and likes looking at pretty women, that doesn't play a part in who he chooses to represent him and his name. I am certain the only woman he "has the hots for" is his lovely wife, the mother of his three children.