Isatis tinctoria
"Indigo dye" is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant ("Indigofera"), or from woad ("Isatis tinctoria"), or the Chinese indigo ("Persicaria tinctoria"). Many societies make use of the "Indigofera" plant for producing different shades of blue. Cloth that is repeatedly boiled in an indigo dye bath-solution (boiled and left to dry, boiled and left to dry, etc.), the blue pigment becomes darker on the cloth. After dyeing, the cloth is hung in the open air to dry.