dyeing grey water, standerds for how much dye to use
Probably enough that your guests can easily see that it's not clear when their dog is looking for a convenient drinking bowl, going for the most obvious inappropriate use of toilet flushing water as potable. Alternatively (and probably requiring a bit more dye) would you notice if you were handed a glass of the stuff that it was blue, or green, not clear?

I assume you'd handle this (and the disinfection) with dosing pumps. Non-toxic dye is available quite concentrated, so it probably won't take much volume. Looking at one example (dye manufacturer's recommended dosing rate), a pound of dry dye treats a million gallons, or a gallon of liquid treats 96,000 gallons (or 12,833 cubic feet, which may help grasp the relation to your current use, depending what your water is billed in.) The liquid would be easier to dose automatically.

You might be able to use a lower rate, but that rate should at least be adequate, and does not seem to imply "blowing through a lot of dye" for remotely reasonable water use. I suspect the standard in play is "does the inspector see that the water is dyed?"