Excluding deffered decals from post process volume?

I’m trying to figure out how to exclude decals from the post process volume. The scene I am working has a PP with a material to desaturate colour to black and white in the array. I’m trying to use decals (mainly blood spats) to guide the player in certain directions while in the level but I need these to still appear red. Usually there would be an option to set custom depth in the render option but I’m guessing that because decals are 2D there is no depth and so it cannot be set this way, would anyone know how this could be implemented?