There are two issues to tackle when it comes to the ruffles on the wedding dress: the color and the shape.
The problem with the color is that due to the changing sunset lighting in the relevant movie scene, the ruffles actually change color as time goes on. They start off looking almost like a rum pink/beige color, but then fade into a purplish-gray as the sun sets.
Given that the lighting starts off rather orange to begin with, it’s tough to tell what the original color was supposed to be. I guessed lavender of some kind, to go with Ursula’s original purple skin and tentacle undersides, but I can’t be sure. Unfortunately, dusty rose/lavender with gray undertones isn’t one of the “standard” colors you find fabric in, and it couldn’t be too shiny or too thin or it would clash with the matte brocade of the dress.
The second issue is construction. Clearly, it would be easiest to just gather long strips of fabric or ribbon, but it seems clear from the movie that these are circular ruffles, which drape nicely but take up a lot more fabric and are seriously annoying to hem due to the curved edge. I thought about using purchased trim, but perhaps unsurprisingly I couldn’t find any wide purplish-pinkish-gray ruffled trim anywhere (or even any white ruffled trim of the correct width that I could dye). So, handmade it is.