I read the OoTP a long time ago and so when I was watching the movie I didn't remembered how the scenes were described in the book
exactly. And to be honest, the fight around the Veil is for me really one of the most impressive scenes of the whole movie. I think the mist, dust, sand or whatever we wish to call it, together with the colors, add just the right dramatic visual element to the fighting and I really love it. The problem of course is that it does not correspond with the text.
As all of you here have pointed out, the sand is an interpretation of Apparating. But then, there is Apparition portrayed in the movie even before, when Fred and George Apparate right behind Mrs. Weasley in Number 12, Grimmauld Place. And
that Apparition is exactly how it is depicted in the books with no signs of sand or additional color, it is just the right pop. So
what are the Death Eater and the Order actually doing in the Ministry in the movie?
-</3-Unseen-to-you-</3, you are perfectly right about the
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flying ability which really is not mentioned until in the very last book. And even in DH it is seen as something so special and extraordinary when Voldemort and then also Professor Snape fly. So it really is not a normal skill any wizard could possess.
So, I would say that the flying was a bit over the top, but I honestly love the visual effects of the mist and colors.
As for the colors black for the Death Eaters and white for the Order, I think it might also have a practical side to it. The battle is rather fierce and fast and the setting of it rather dark, so it actually helps the viewers follow who is who and where at the moment; in other words, which side is winning and which is losing.
How does that sound?

~Jana