i need to do a blood bath for one of my movie scenes, but am struggling with what substance to use for the blood. i have used food colouring in water for a previous scene and it didn't stain the cup, knife or counter. but will it stain the bath tub? should i put a large amount of water?

i don't want to fill the entire bath tub, coz an actor will be sitting in it. i just want like little drips outside the tub.