Hello seems like a good idea. I will let you know here because it's very simple, I've tried it once and worked perfectly.
Create a Hue/Saturation adjustment layer and bring the saturation to -100, duplicate this layer and convert the blending mode to screen, keep duplicating the screen layer until your dress turns white; this depends on its original colour. Remember to mask all layers to the layer of the dress.
I hope I helped! Let me know if you faced any problems concerning this.
How do you cut the dress to a single layer? Surely the selection tool or whatever would bake a mistake along the way, then i would be turning the skin to that colour too, and it'll probably be obvious its Photoshopped. This technique would probably be impossible to use on a photo, although a CG drawn image would already have everything separate in its own layers, so that would be a piece of pie :/
I usually cut with the pen tool..As for the image obvious as photoshopped i don't know if you've seen the result of this image, because it doesn't seem unnatural..Also If you're a photoshop pro you would know that anything cut can be fixed later on, it's not a big deal