Swapping landscape textures in a specific area??


I’m making a level where as the player unlocks areas of the map, that area transitions from black/white to color to show that is has been unlocked. I want the player to be able to see a line on the ground merging black/white and color between areas that have been unlocked and locked.

The problem is I’m using landscape and foliage tools to make the whole map and since I want to be able to physically see where the unlocked area ends and locked area starts I don’t think post processing will do the trick. Is there a way to change all textures in a specific area (landscape and foliage) to black/white then swap to color? I was thinking of maybe using masks to tell landscape where locked areas are but no idea how to implement it in the shader?

Any ideas???