Yes it’s the Great Hall clip. It’s so brown you could mistake it for a flashback.
The thing is that realism is, to certain degree, subjective. I don’t see Prizoner of Azkaban as any more realistic than Chamber of Secrets. Thus, I thought of the most prominent change - the color palette - and assumed that must’ve been what you were referring to. I see people (falsely) equating grit to realism all the time, so I figured you were doing just that.
That said, I should’ve thought of the costuming. I can definitely how you would consider the post-Columbus films to have more grounded, realistic costumes. The early films have wacky wizard robes the later films have students often wearing muggle clothing (a pretty big and easily avoidable plot hole, IMO). On a more subtle level, the way the uniforms look changes from uniform (no pun intended) and well-kept to messy and personalized. As usual I prefer the lighthearted unhinged fanytasy of the Columbus films for this, but here my opinion isn’t strong, as I’m not to bothered with either style
Consistency on par with LotR (which it had because it was shot all at once with the same production team) is definitely a good thing strive for.