So what I want to do it as you go down on health your screen goes down on colour to a grey scale sort of thing.
You csn alter rendered pixels via Post Processing (which is shader applied to the screen… yes you can apply material to screen read Post Processing Material for that) and UE4 has lot of build-in default effects, and what you looking for is professionally called Color Desaturation and you can do that by decreasing Color Saturation. Here you have docs about Post Processing, there should info how to use it: