Hi Folks,
I've only just seen this thread (and have only scan read it) but thought I'd post the following quickly in case it helps
An interesting note -- the Voldemort costume on display looks green. It is in fact black silk, but they threw a pale green drape underneath it because the black bodysuit Fiennes wears looks like Voldemort Men in Tights when its on display.
Unfortunately that's not strictly true. The robes were actually green although in most lights they do look admittedly look black.
For GoF they used over 50m of silk :eek.
The silk used in GoF was very thin, lightweight to give that "whispy" effect that you see as he's resurected. It works beautifully with the CGI transition.
For OotP they changed the robes to use less fabric (although it's still a lot!). Again, they stuck with silk (2 types in fact), but they switched to a slightly denser weight.
The theory is that some time had passed since he was resurected so he'd gained in strength so this was reflected in a more "Solid" looking costume.
Also, the robes are made of 2 parts - the outer bit which "flows" and an inner robe. Again, both are green.
If you check out the Deathly Hallows trailer (I've not seen the film yet so I'm not sure if it's seen in the 1st one), the robes have remained the same in shape although there is a change in one aspect......I won't say what or why though yet as it's a potential spoiler
Also, just to clear up the Snape colour debate. They're actually blue, although to be fair, if I didn't know better I'd also have said they were black too :lol (and that's the way it should be - you'd want Snape to be wearing black after all wouldn't you!)
Apparently the problem is, if they'd have used black they (on screen) would have lost lots of the detailing (especially in scenes with filters applied) so it would have ended up looking really flat and naff.
I guess the key thing with that is, from a costuming point of view, if you feel blue wouldn't look right, there's no harm in going with black.
I hope that helps.