I have two possible picks, they’re very different movies so I think I’m safe: “Blue” by Krzysztof Kieślowski and “Suspiria” by Dario Argento.
I think a common theme is that both films used very bold use of color. With Suspiria that is quite obvious, it’s almost psychedelic in its use of color. Blue is more subtle, but it rings true to its name. Watch it a dozen times like I have. You will see it.