Reproduction: Create a resource pack that replace assets/minecraft/textures/block/black_shulker_box.png into the texture of brown shulker box, and replace assets/minecraft/textures/block/red_shulker_box.png into the texture of blue shulker box, then applying the resource pack and observing the color of black shulker box and red shulker box.
Expectation: Black shulker box would turn brown and red shulker box would turn blue.
Observation: Black shulker box still black and red shulker box still red.
black_shulker_box.png with brown’s texture
red_shulker_box.png with blue’s texture
won't change how the actual block appears in-world because the game ignores those textures for in-world rendering and uses entity-based rendering instead (shulker_box is technically a block entity with special renderer code).