The original craft of indigo dyeing was a manual and messy business. Filthy water, dye sludge, and scum residue are all bi-products of the hand dye process.
The water residue from natural indigo hand dyeing takes on a greenish cast whereas the dye itself seems to cast reddish. Such is the nature of natural plant derived indigo. Modern chemical indigos are more controllable but nevertheless the locality where it is used has an effect on the cast due to the PH in the water supply.
The craft of hand dyeing seems romantic but in the case of indigo the ultimate contamination was and still is a major pollutant to the environment. In the past indigo artisans didn't care about cleaning the old water before throwing it onto the fields whereas now it is global law to clean up the water to at least the same level of cleanliness as when it was extracted from river or stream. In fact many modern industrial mills have an enclosed recycling system for their dirty water, cleaning it and then re-using to dye again. When the water has been used too much it is then cleaned to a higher level of purity before being discharged into the river or stream from whence it came.