About Prisoner of Azkhaban, I certainly agree Alfonso Cauron made an excellent movie, and he made a good choice with the tone. However, I think people are a little unfair in believing it was all Alfonso.
The way I look at it, the first two movies/books, are when Harry is new to the Magical World. All the small feats of magic are front and center with a spotlight on them, the colors are bright and vivid, the streets and the walls of the castle are pristine, etc, etc...because we're seeing the world through Harry's eyes, and it's all new to him, he's young and Naive (11 and 12 Years old).
By the Thrid movie, the colors are toned down, the little feats of Magic are moved into the background, the streets are dirtier, the castle walls are marred from 1000 years of underage magical abuse. This all, to me, represents that Harry is now more familiar and acclimated with the Magical World, it's no longer all a big amazing wonder, he no longer naively sees things so perfect, he can see the details now.
The transition from the first two movies/books, is similar to when you move into a new house or apartment, you think it all looks so fresh and new at first, and after you've been there for awhile, you see that crack on the base board, or ugly cupboard handles, or the paint glitch in the corner of the living room, etc.
So, like I said, I don't take anything away from Alfonso's movie, he did a great job, and Columbus, very well may have screwed it up, and kept it in the same tone as the first two, but, I do believe the text of the third book neccesitated this change in tone.