mobile friendly heat vision

hi. is there any mobile friendly way to create a heat vision effect? like when it’s on i want my enemies to be black(or white) and the rest of the world to be white(or black! doesn’t really matter!) i am thinking about changing the materials but i guess it’s not very mobile friendly?! and my world is static so i guess i can’t change their materials anyways! right?! i’m not sure about post process too! want it to be as light as possible!

we are a sniper and we want these soldiers to be more visible when enabling this effect. and if there is any other way to highlight the enemies i’m open to suggestions! it doesn’t necessarily have to be like a heat vision!