"Indigo dye is an organic compound with a distinctive blue color (see indigo).
Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today — several thousand tons each year — is synthetic. It is the blue of blue jeans."
"Indigo is a dark blue crystalline powder that sublimes at 390–392 °C. It is insoluble in water, alcohol, or ether but soluble in DMSO,chloroform, nitrobenzene, and concentrated sulfuric acid. The chemical formula of indigo is C16H10N2O2."
"The molecule absorbs light in the orange part of the spectrum (λmax = 613 nm).[8] The compound owes its deep colour to the conjugation of the double bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is nonplanar."