There is not a universal agreement , however , on the number of colours in the rainbow .
According to his publication , Optical Lectures ( 1672 ), when Newton refracted the light with his prism , he initially saw five colours in the spectrum ( red , yellow , green , blue and violet ). By combining red with yellow and red with violet , he then added orange and indigo to the spectrum , for a total of seven colours . In 1704 , following Newton ’ s publication , Optiks , ROYGBIV became universally accepted as the seven spectral colours of the rainbow .
One hypothesis put forward to explain his addition of orange and indigo to the spectrum is that Newton was infatuated with the mystical connection between numbers and reality , in particular the number 7 and its link to the mathematical patterns in music .
The Gay Pride flag forgoes indigo in its representation of the rainbow . Although the flag made its debut in 1978 with eight colours , the traditional and most common variant consists of six stripes .
Pink Floyd ’ s classic album cover for Dark Side of the Moon features a conceptualized depiction of Newton ’ s prism experiment . Of note , the band of light emanating from the prism only has six colours - it is missing indigo .
Three centuries later , modern portrayals of the rainbow have dropped indigo , thereby reducing the number of colours in the rainbow to six !
Considering that few people can differentiate the wavelengths well enough to see indigo separately from violet , dropping it from the rainbow seems to have been a sensible turn of recent events . Also , the six-colour spectrum visually correlates with the current model of the colour wheel : red , orange , yellow , green , blue , and violet .