Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours.

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1. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal mixture of red and green light. On a colour wheel based on traditional colour theory (RYB) where blue was considered a primary colour, its complementary colour is considered to be orange (based on the Munsell colour wheel).
2. The English language commonly uses "blue" to refer to any colour from navy blue to cyan. The word itself is derived from the Old French word bleu.

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• The modern English word blue comes from Middle English bleu or blewe, from Old French bleu, bleve, blöe, a word of Germanic origin (Frankish or possibly Old High German blāo, "blue").
• Bleu replaced Old English blāw "blue" and blǣwen "light blue". The root of all these variations is Proto-Germanic blǣwaz, from Proto-Indo-European *bhlāw-, *bhlēw- "light-coloured, yellow, grey, blue", from *bhel- "to shine, be light or bright", also the root of Old Norse blār and the modern Icelandic blár, and the Scandinavian word blå, which can also refer to other non blue colours.