This is a simple plugin for the addon Bagnon that first and foremost shows the item level of your equipable gear, but also additional information like the bind status of items (BoE, BoU), a purple eye on uncollected transmog appearances and desaturates and tones down vendor garbage.
Unlike it's similarly named sibling Bagnon ItemLevel which is mainly a straight forward one-trick pony for people only needing the item levels, this addon has an options menu and gives you far more control over the various settings.
To configure settings like whether BoE- and item levels should be rarity colored or how much darker garbage should appear, go to the Bagnon Itemlevel Plus entry in the blizzard interface options addons menu, or open the options menu directly in its own window with the command /bil.
I recommend downloading the addon from either CurseForge or Wago, or use their respective download clients which provides the same versions and installs them automatically for you. Be aware that only these download locations provide finished packaged addons with all libraries included.
If you choose to download and manually install directly from GitHub, you'll have to install the following libraries as standalones in order for the UI to work: