Don't do red, red is far too difficult to concentrate into a cake. It would take at least 50g of gel to cause that effect on someone.

Your go to is Blue, Green, or Yellow (though yellow won't be as hilarious)

Go to a specialty food store and look for Galloway or Ess brand colors, they are highly concentrated and will dye that poo up something fierce. 


Source: I bake and shit. 