The preview window border usually matches the color of the Shulker Box (e.g., a yellow box has a yellow tooltip).
While primarily client-side, installing it on a server enables extra features like Ender Chest previews. Configuration and Links ShulkerBoxTooltip
Hover over a Shulker Box in your inventory and press a modifier key to see a window showing exactly what is inside. Customizable Layouts: Shift (Default): Opens a compact preview of the items. The preview window border usually matches the color