ItemHighlighter features

Your color, on your items, everywhere you look at them.

Where highlights show up

Hotbar and offhand. The held item textures get your exact color while no menu is open.
Every container screen. Inventory, chests, furnaces, the creative menu, and the cursor-carried stack all highlight the same way.
Tooltips. Hovering a highlighted item anywhere adds a colored ★ Highlighted line. Rarity colors and other lines are untouched.
The world. Dropped items and item frames holding highlighted items get a real glowing outline using vanilla's own glow rendering. The world only supports the 16 palette colors, so the outline uses the nearest one; your exact color still shows in the HUD, the tooltip, and /hl list.

The inventory with tinted diamonds and emeralds and a tooltip reading Diamond, ★ Highlighted. — Inventory: tinted textures and the ★ Highlighted tooltip line.

A chest full of mining loot where only the diamonds and emeralds are tinted. — Chests too: spot the diamonds in a pile of loot.

Dropped items glowing in their highlight colors. — In the world, the outline uses the nearest of the 16 palette colors.

The texture, not the slot

The highlight hugs the item's opaque texels: the tint lands only where the sprite is solid, a crisp border sits on transparent texels directly touching the item, and one soft halo ring sits a texel further out. Empty sprite corners are never painted, so a diagonal sword gets a diagonal glow instead of a box. Items whose shape cannot be read (3D models) fall back to a tight sprite-edge border.

Per type by default, exact if you ask

Per type. Highlighting a diamond sword highlights every diamond sword (matched by registry id, e.g. minecraft:diamond_sword).
Exact. With matchComponents enabled, only stacks identical to the held one match: same enchantments, custom name, damage, and other components. Your named Fortune pickaxe highlights without lighting up every plain diamond pickaxe. Items with no extra components behave the same in both modes.

Highlights persist

Highlights live in config/itemhighlighter_data.json (up to 256 items, oldest dropped first) and survive restarts. Entries created in one matching mode only match in that mode, except component-less items whose keys are identical either way.

What it never does

It does not rename items. /hl &a&lMy Axe highlights the held item green and ignores the rest; the item keeps whatever name it had.
It does not change items. Counts, damage, enchantments, and NBT are never modified, on any side.
Nobody else sees it. The mod is client side and per player. Other players never see your highlights, and servers need nothing installed.

Performance

Item shape masks are resolved once per item type and cached. The world glow syncs once per client tick and restores anything it touched when highlights are removed or disabled. Every render and tick hook is guarded so a failure can never crash the frame.