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.

Despite 24w44a unifying empty slot textures by moving them all to the one directory, the colors are still inconsistent.

Slot colors

Slot	Color
amethyst_shard	85 85 85
axe	85 85 85
banner_pattern	58 58 58
banner	55 55 55
boots	85 85 85
brewing_fuel	104 104 104
chestplate	85 85 85
diamond	85 85 85
dye	55 55 55
emerald	85 85 85
helmet	85 85 85
hoe	85 85 85
horse_armor	124 124 124
ingot	85 85 85
lapis_lazuli	85 85 85
leggings	85 85 85
llama_armor	124 124 124
pickaxe	85 85 85
potion	104 104 104
quartz	85 85 85
redstone_dust	85 85 85
saddle	124 124 124
shield	85 85 85
shovel	85 85 85
smithing_template_armor_trim	85 85 85
smithing_template_netherite_upgrade	85 85 85
sword	85 85 85
Programmer Art horse_armor	124 124 124
Programmer Art lapis_lazuli	84 84 84
Programmer Art potion	104 104 104
Programmer Art saddle	124 124 124

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. A zip file is attached containing slot textures recolored to use color 85 85 85. Mojang have my full permission and encouragement to use these as the fixed textures.