Colour World with Actual Material and Decals

In the game, the world is entirely white. The main objective of the game is to fire paint to colour in the world again where that paint lands but I have not worked out how to do that yet.
As I understand it, the materials should have a white mask over it, then the paint decal should “burn” through it, revealing the world’s true materials e.g. green if grass, red wood material if it hits the side of the temple etc.
Unfortunately, I have no idea on how to do either! Any help would be greatly appreciated :slight_smile:

Thank you in advance.