Yes, that’s the palette look-up texture. It is for a simpler & faster palette-changing shader. Horizontally are laid out colors the gray value is pointing to. Vertically you can change the active palette. I don’t know about Unity shaders, soon I might be able to create an example for it. Meanwhile, I have provided a link to example projects in the description for GameMaker2 and Godot.
its possible to export edited Sprite sheet with color? Because i see only "Save gray Sprite"
It was made as a tool to generate textures for shaders. I see that it could be used for recoloring although there are better tools for that. I use Aseprite or Libresprite for that. But I’ll put that feature in fairly soon.