Several textures for slots that indicate what items can be placed inside them include a color or the entire slot texture as a background instead of having a transparent background like the armor slot textures for the player inventory. These textures include (all are located in assets/minecraft/textures/gui/sprites/container):

• horse/armor_slot.png
• horse/llama_armor_slot.png
• horse/saddle_slot.png
• loom/banner_slot.png
• loom/dye_slot.png
• loom/pattern_slot.png