Iron is added to a dye bath to 'sadden' the color, creating darker shades and sometimes completely altering the original color. Photos below demonstrate fabric before and after using an iron spray solution.
Iron can also be used as a mordant prior to or post dyeing.
Instructions included Use about 1% iron to weight of fiber. Keep out of reach of children! Iron is dusty and can irritate skin, eyes, and throat. Wear protective gear and use caution with these ingredients.