The main difference between Blue and Indigo is that the Blue is a color; additive and subtractive (RYB) primary color; visible between purple and green and Indigo is a deep and bright shade of blue.
Blue is one of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. It lies between violet and green on the spectrum of visible light. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colors; azure contains some green, while ultramarine contains some violet. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called Tyndall scattering explains blue eyes. Distant objects appear more blue because of another optical effect called atmospheric perspective. Blue has been an important colour in art and decoration since ancient times. The semi-precious stone lapis lazuli was used in ancient Egypt for jewellery and ornament and later, in the Renaissance, to make the pigment ultramarine, the most expensive of all pigments. In the eighth century Chinese artists used cobalt blue to colour fine blue and white porcelain. In the Middle Ages, European artists used it in the windows of Cathedrals. Europeans wore clothing coloured with the vegetable dye woad until it was replaced by the finer indigo from America. In the 19th century, synthetic blue dyes and pigments gradually replaced mineral pigments and synthetic dyes. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits. Because blue has commonly been associated with harmony, it was chosen as the colour of the flags of the United Nations and the European Union.Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and sometimes with sadness. In US and European public opinion polls it is the most popular colour, chosen by almost half of both men and women as their favourite colour. The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm and concentration.
Indigo is a deep and rich color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine. It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum. The color indigo is named after the indigo dye derived from the plant Indigofera tinctoria and related species. The first known recorded use of indigo as a color name in English was in 1289.
From Middle English blewe, partially from Old English *blǣw ("blue"; found in derivative blǣwen (“bluish”)); and partially from Anglo-Norman blew, blef (“blue”), from Old Frankish *blāw, *blāo (“blue”) (perhaps through a Medieval Latin blāvus, blāvius (“blue”)); both from Proto-Germanic *blēwaz (“blue, dark blue”), from Proto-Indo-European *bʰlēw- (“yellow, blond, grey”). Cognate with dialectal English blow (“blue”), Scots blue, blew (“blue”), North Frisian bla, blö (“blue”), Saterland Frisian blau (“blue”), Dutch blauw (“blue”), German blau (“blue”), Danish, Norwegian and Swedish blå (“blue”), Icelandic blár (“blue”), Latin flāvus (“yellow”), Middle Irish blá (“yellow”), Lithuanian blãvas (“blue”). Doublet of blae.
blue (comparative bluer, superlative bluest)
blue (countable and uncountable, plural blues)
blue (third-person singular simple present blues, present participle blueing or bluing, simple past and past participle blued)
From the color of the envelopes used to contain missives of the censors and managers to vaudevillian performers on objectionable material from their acts that needed to be excised.
blue (comparative more blue, superlative most blue)
16th century (as indico, modern spelling from the 17th century), Spanish índigo, Portuguese endego (modern índigo), or Dutch (via Portuguese) indigo, all from Latin indicum (“indigo”), from Ancient Greek Ἰνδικὸν (Indikòn, “Indian dye”), from Ἰνδία (Indía).
indigo (countable and uncountable, plural indigos or indigoes)
indigo (comparative more indigo, superlative most indigo)