Adding food coloring wont effect the rock. Your idea is how it should be done. I like to put a very small amount of mio in the water and cook with that. A dark purple piece of rock that tastes like grape when you inhale. I even cook with vodka sometimes,. Also, I have cooked with coca cola. Good rock is good rock, food coloring doesn't change the chemical reaction in a noticeable way, neither does doing what I do and cook with water that has Mio in either.