The movies are marketing tools, they have absolutely nothing to do with the original works. They're adaptations, just as the Narnia and Lord of the Rings movies were. If I hear someone else complain about the style/color of a freaking dress one more time within the fandom, I'll cut off their head and use it as a doorstop. Seriously, being this irrational over Hermione's dress just isn't worth the time and effort it's given. This rant also goes for the nitpicking over Harry's shaggy hair in the fourth film, the outfits worn for the Triwizard Tournament, Dumbledore's wardrobe (which I still love, by the way), and all the other pointless shit people try to nitpick just to waste time.
Personally, I adore Emma/Hermione's dress for the Yule Ball. Pink is more fitting on her than blue, in my opinion. The pink collaborates with her complexion and the flushing of her cheeks, and it's an elegantly looking style of a dress -- she reminds me of Kaylee in this shot from the episode "Shindig". Only less fluff and old-fashioned style.
In conclusion, I adore the dress. Of course, I'm not that kind of person that would look for imperfections of the HP films and find the mistakes about what is or isn't included in them. Because, as we all know, the color of a simple Yule Ball dress is incredibly vital to the plot of the fourth book, and changing it would be a crime against the entire series. God forbid they try to make it a little different than the book itself, never mind the fact that months ago the fandom was frightened and fuming that they might cut out the Yule Ball scene.
Just had to get that out. It's been irritating me ever since the promo picture was released, thus releasing the havoc and whining amongst the fandom.