I didn't read the books.
Prisoner of Azkaban— and to a lesser extent Chamber of Secrets and Goblet of Fire— had a necessary mix of whimsy, humor, color, and charm, and also seriousness/darkness. The series should have stayed that way.
But then first they sucked out the literal color, then the literal light, then the happiness out of the movies. By the time I saw the earliest part of Deathly Hollows, I just stopped caring. Every moment became joyless and hard to watch.
If there's no more discovery of the whimsy of the wizarding world, no more wonderment, no more seeing characters be just plain likeable, why do I care anymore? The series slowly became about friends being jealous and petty, people being double-agents, and about death and loss.
I had to read a summary of the events of Deathly Hollows, because I couldn't subject myself to 4 more hours of misery after the latter two-thirds of Half-Blood Prince. Now I'm just going to enjoy the full LEGO Harry Potter collection game.
They aesthetic choices of the movies directly reflect the events of the story. Voldemort coming back into power literally drained all the joy out of the wizarding world for those who knew and understood what was happening. The story turned from one of discovery to one of self preservation.
Voldemort let dementors loose to feed on any joy they could find, no wonder the characters became jealous and petty. Then the sense of impending doom from the threat of Voldemort or his followers and their terrorist style attacks meant that any gathering that could have been joyous turned downtrodden and melancholy.
The tone of the films reflect these events. The films didn’t touch on the prevalence of the dementors except briefly, so instead they showed the constant cloudy grey skies. The sense of impending doom is seen in the darkness of the scenes. In settings where the characters can be more free or relaxed the scene is brighter and more colorful. In settings where that relative safety is more easily broken the scene is dark and foreboding.
I’m not trying to say the movies were all perfect gems that got everything right, but the stylistic choices for scene and lighting I think were used to emphasize the greater tone of the events as seen by the characters.