How does fog of war works exactly?

[Narvana]

I would want to mod visible parts of the map, so they would use fog of war parchment graphic style. But I'm not entairly sure if that's possible.

My question is, how FOW works exactly? Is this a filter covering normal, colorful map? Or does hexes use another textures? Can I simply swap them in ArtDefs folder to make it work?