Princess1, I'm not sure why, but you're question about the Rainbow dress got my brain a-churnin' today. **laugh** I've been thinking about it a lot, and I think I see how Biggars and co. pulled it off. Hopefully this will give you some ideas.
**warning: this is long**

First, yes, it's definitely going to be a custom dye job. But not unlike the "flame" gown the handmaidens wore in Ep. 1. In fact, you could probably achieve that look using the same technique. Probably. Mind you, I don't do a lot of fabric dyeing (Rit is the height of my experience), so I could be wrong. I have also seen some dresses out there lately that are doing the two-colors-fade-into-each-other look. So it may be possible to find fabric already dyed this way.

Next, the dress top appears to have two layers. The first layer (underneath) looks like a light yellow spandex-type fabric done in halter-top style. It comes up to that point just below her neck and attaches to the choker. There isn't a visible seam for a zipper in the back, which is why I'm thinking it's spandex or stretchy spandex/cotton leotard fabric (it kinda has that spandex-y or lycra shimmer to it too). Also, the seam(s) is/are probably either at her sides or front, hidden under the draped fabric.

The skirt of the dress is probably two parts as well. I'm going to throw my lot in with a basic circle skirt done in multiple layers of a light-colored light-weight fabric (like lining for wool slacks and blazers - I forget the actual fabric name ^^; ).

The top layer is made from the dyed chiffon fabric. It looks like a heavier chiffon, or else it was doubled before it was sewn in place. This was draped around the "under-dress" for the effect seen in the pics. The fabric starts (obviously) at the choker, and looks like it's sewn to about the half-way point at the sides of the spandex "undertop." The sleeves are gathered together all the way up and attached to the silver armbands, and then the loose fabric from the neck is gathered into the sleeve (in the same seam) a few inches below the armband - both on the "top" of her arm and "underneath" to create the flowy sleeve effect. ... It also looks like it was sewn on in an upside-down triangle - which is why it comes to that knotted "point" in the front. The top layer of the skirt is pulled up in a bunch at the waist in back - kind of like a bustle.

The armbands and choker could be done quite nicely using Sintra (see http://www.azeem2000.com/ for info on sintra).

**reads through description**

Good grief! I hope some of that makes sense! O.o;; If my rambling thoughts seem like they're way off in left-field, here's the pics I was using for reference (courtesy of Réka):

Rainbow dress

Maybe that will help clear things up. If you have any questions, just ask, and I'll do my best to clarify myself.

kreleia ^.^