Any advice on how to do the light/dark contrast on the folds? Maybe an example would be good. I think I can do the saturation by myself.Not bad at all.
When 'cutting out' be sure to soften the edges of the shadows since they became hard when doing the selection.
For that local work you can use the soften tool.
There's also a lack of light/dark contrast in the folds of the new dress and the colors are a bit too saturated since the sunlight is coming from the top right of the image and most of the dress isn't exposed to sunlight.