I've just reached certain episode yesterday, so seeing this in retrospective is kinda painful.
Almost sure Dezaki's style was influenced by film. But if that were the case, he didn't limit himself to replicate it's techniques, he also used the unique resources of animation to create even more powerful images.
But as we meet the more “damaged” members of the main cast, the colors become darker. This amidst a storm, which should make things clearer.
Something striking about this first episode is how the intro scenes use a pastel tone palette to convey a dreamlike quality that perfectly reflects Nanako’s naïve expectations.
Apparently, Omata Shinichi's largest contribuition at SHAFT, was his work on Arakawa Under the Bridge as storyboarder (1,2,5,11,13,18,22) and episode director (2,11,15,22) under Yukihiro Miyamoto's direction. Both worked together in Hidamari too.
There's a segment on Arakawa Under the Bridge's second season where they parody "Aim for the Ace" and it's director and storyboarder, Shinichi Omata a.k.a. Mamoru Hatakeyama (the director of Rakugo/Kaguya-sama) goes full Dezaki.