The bug

Icons used for empty slots are not all shaded consistently. Multiple different colors can be seen to be used across different such outlines.

How to reproduce

Open the following inventories, and note how dark or light certain empty slots are, specifically how they aren't all the same:

• Player inventory - armor slots, offhand
• Horse - armor, saddle
• Llama - carpet
• Brewing stand - bottles, fuel
• Enchanting table - fuel
• Smithing table - all three input slots
• Loom - all three input slots

How to fix

This could be fixed by just retexturing these slots to make the color used consistent.

[OUTDATED BELOW]

Attached, however, is a resource pack, which splits off these icons from the inventory panels themselves (as per MC-74408), as well as making them all use the same color scheme as the player's armor slots and offhand do. These icons would be made to be overlaid on these inventory slots, rather than being baked in, to fix issues like them showing up behind items when they shouldn't. All of these icons are moved to minecraft:textures/slot/ (including the five existing inventory icons, which as of 21w37a are still in the items directory, despite not being item textures themselves), and could be compiled into a dedicated texture atlas (minecraft:textures/atlas/slots.png-atlas?) (if the blocks and items atlas isn't sufficient itself).

Note that the resource pack doesn't make this change fully, and just makes the respective inventory slots blank so that a code change can overlay the newly split sprites.