A shader that uses vertex colors on a model for its lighting.
Normally you should download this from asset.party for s&box as a .shader_c. However the source here is to allow others to contribute or make adjustments for themselves.
Move the .shader into any addon folder you like, excluding s&box's default addons, so make a new one if you have none. Preferably you should make a shaders folder inside of that addon folder if you made a new one, or add that as a folder to the root of an existing addon.