Although traditionally considered one of seven colors of the rainbow or the optical spectrum, modern color scientists do not usually recognize indigo as a separate division and generally classify wavelengths shorter than about 450 nm as violet. Indigo is a color named after the blue dye derived from the plant Indigofera tinctoria and related species. Indigo is between blue and violet (http://en.wikipedia.org/wiki/Indigo_(color).
The Indigo color wheel has darker, stronger colors, some call them “loud colors”. Many fashion statements can be made by using colors from the Indigo color wheel. Indigo is commonly connoted as loyalty, religion, spirituality, & intuition.