I am looking into grey water systems for a future house. In the course of researching, I learned that in states that have adopted the International Plumbing Code, you can use the water for flushing toilets as well as irrigation. If you use it for flushing toilets, you have to dye it with either blue or green food dye, as well as disinfecting it in some way.
My question is: Is there a standard for how much dye to use per volume of water or a standard for opacity or something? I am mainly concerned about blowing through a lot of dye to make the water colored.
I have done lots of research on this, but all the documents i have found merely state that the water has to be dyed, not by how much.