Globally desaturated level with saturated spots

I have a top-down god game and I’m trying to create a level which is globally desaturated in the beginning. When the player conquers a spot, the spot will become saturated. How can I achieve this?

I’ve tried putting post process components on the actors that populate the level, with saturation set to 0 in the beginning and then switching to 1 when they are conquered, but I can’t get it to work. It’s always completely desaturated or saturated, no matter how much I play with blend radius, priority etc.