Called “green vitriol” in old dye books, this is an essential of the dyer’s art. Iron after baths can take any yellow or gold and turn it to a soft green. Reds become burgundies, pinks become plums, and any tannin source (any weed or bark) can be used to make pretty grays, just by adding iron.
Usually used as an after-mordant modifier, where you just watch the colour and when it has changed “to taste”, rinse it. If used as a pre-mordant, ½ oz per pound is the usual ratio = 3%. Do not use too much as it can “weight” and damage delicate fibers.