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It is COLOR_shulker_box, so for example yellow_shulker_box

Be aware that the light gray shulker box has the id silver_shulker_box

minecraft:black_shulker_box
minecraft:brown_shulker_box
minecraft:blue_shulker_box
minecraft:light_blue_shulker_box
minecraft:yellow_shulker_box
minecraft:orange_shulker_box
minecraft:magenta_shulker_box
minecraft:purple_shulker_box
minecraft:red_shulker_box
minecraft:pink_shulker_box
minecraft:gray_shulker_box
minecraft:silver_shulker_box
minecraft:white_shulker_box
minecraft:lime_shulker_box
minecraft:green_shulker_box
minecraft:cyan_shulker_box