Need to fill bathtub full of blood for a scene

Ok so I am shooting a scene where the camera dollies up to a bathtub where the actor is already inside and the bathtub needs to be full of blood and his head emerges from one end, The tub is porcelain and It absolutely cannot stain the tub. Also I want the blood to be not so much on the bright red side but more of a darker/blackish red just to add to the tone of the film but also safe for my actor to be in as I was reading using some paints is a better option than making the blood with food coloring, Any help would be greatly appreciated!
 
I spent many years in make up efx so it won't be a surprise to
hear that I have done just that dozens of times. Use tempera
paint - water soluble and doesn't stain.
 
Great advice above.

You might find this helpful as well:

https://www.youtube.com/watch?v=Db7AJGlvuiA

Film Riot - Bipolar Josh and Six Ways to Make Fake Blood!
https://www.youtube.com/watch?v=A-gH7f7mQoY
 
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