Help with photo manipulation

Hey all,

This beautiful collage was made by Luis Dourado and I am trying to understand how he made the dress part of the background. As you can see, the dress is still there, he did not delete it, rather he some how removed the color but saved the dress' texture and shades. Any advice on how to do this?

I am trying to to the same with the following dress and I want to keep the textures of the fabric (like where the fabric folds or where there is a knot).

will appreciate your help!