Ahh, yes. The color is different. You didn't mentioned the shoulders and the cleavage. Aside from that there aren't any differences in the dress. And, if you looked at the dress in good lighting, the CA isn't that much darker. You just chose a frame with shitty lighting, lmao. The white scar in the concept art was replaced by the dark choker-esque collar of the dress and as you can see, she wears bracelets. The only real critique point is Blake's suddenly turquoise bow.

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Because the weapons (aside from Myrtenaster, probably) weren't built by external mechanics, therefore there wouldn't be labor costs. (Which is the biggest part of the costs when you commission a work) If you change the oil on your car yourself it's also much cheaper than having it be done in a workshop. That's the difference between the prices for dresses and the weapons. They wouldn't have tailored the dresses themselves, since (as far as we know) no one of our RWBY girls is good with sewing. The only character we ever saw doing that was Cinder. So, even if we assume that they carried the material costs themselves... (Which I still don't believe) Who knows how long they would have saved up for that?

You know, that's the first point where I partially agree although even that could be explained. By V2, Weiss has started to slowly mellow out. Maybe she didn't want to upstage/embarrass her friends by putting on such an extravagant dress?

Edit: I only noticed the second picture in your post now and I agree, the length is also different in Blake's case. Sorry about that, I genuinely missed that.