Indigo

Indigo is a term used for a number of hues in the region of blue. The word comes from the ancient dye of the same name. The term "indigo" can refer to the color of the dye, various colors of fabric dyed with indigo dye, a spectral color, one of the seven colors of the rainbow as described by Sir Isaac Newton, or a region on the color wheel, and can include various shades of blue, ultramarine, and... (Wikipedia)