Recently I learned that there were only four colours in rainbows, in medieval times. Then Newton came along and decided there were 'as many colours in a rainbow as there are notes in an octave'. This gave us the spectrum we have today.
> For it is of four colours, and takes its appearance from all of the elements into itself. From the sky it draws the fiery colour, from the waters purple, from the air white, and from the earth it gathers black.
Source: https://forthewynnblog.wordpress.com/2017/11/20/how-many-col...