From Greek 'indikon' (the Indian substance) — the deep blue dye that India exported for millennia, named after its homeland.
A deep blue colour between blue and violet in the spectrum; a tropical plant of the pea family, from which this dye was originally obtained.
From Spanish 'índigo' or Portuguese 'indigo,' from Latin 'indicum' (Indian dye), from Greek 'indikón' (Indian substance), neuter of 'Indikós' (Indian), from 'Indía' (India). The word literally means 'the Indian thing' — a reference to the Indian subcontinent as the primary source of the blue dye extracted from the indigo plant (Indigofera tinctoria). India exported indigo to the Mediterranean
Isaac Newton included 'indigo' as one of the seven colours of the rainbow — red, orange, yellow, green, blue, indigo, violet — partly because he believed seven was a more harmonious number than six (he was influenced by musical theory and wanted the colour spectrum to correspond to the seven notes of the diatonic scale). Many modern colour scientists consider indigo unnecessary as a distinct spectral colour, arguing that Newton's seven-colour spectrum reflects numerological preference rather than perceptual reality.