Since I don't as yet have gels and a background light I thought I'd experiment with hue>saturation>colorize. I selected the background (Select>Color Range, and then Clicking on the Quick Select tool to get the Refine Edge option)and then went to Hue>Saturation>Colorize and moved the Hue Slider, followed by a move of the Lighten/Darken slider till I got what looked good to me. There was a little spill of the blue onto the cloth beneath her so I used the clone tool to cover it.