<P> The Early Modern English word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic color - naming system . Modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers, which lies on the short - wave side of color wheel (RGB) blue, towards (spectral) violet . </P> <P> However, the correspondence of this definition with colors of actual indigo dyes is disputed . Optical scientists Hardy and Perrin list indigo as between 445 and 464 nm wavelength, which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long - wave side, towards azure . </P> <P> Isaac Newton introduced indigo as one of the seven base colors of his work . In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England, supplanting the homegrown woad as source of blue dye . In a pivotal experiment in the history of optics, the young Newton shone a narrow beam of sunlight through a prism to produce a rainbow - like band of colors on the wall . In describing this optical spectrum, Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations ." He linked the seven prismatic colors to the seven notes of a western major scale, as shown in his color wheel, with orange and indigo as the semitones . Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall: </P> <P> I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it . And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks . </P>

When did indigo become a color of the rainbow