Rochelle fabric on the skirt and in the new MS Ghoul's Rule movie is this stuff with the black painted on.
My guess is that they have white underslip for the skirt to make it pop more with the rainbow effect.
Because you can't iron metalic or nylon. I recommend using Jacquard Airfix to set you paint by mixing it into to your fabric paint.
(can be thinned out)
Found this on spoonflower if you wanted to skip the mirror finish. BUT the size of the pattern is only 14" tall. >_<