"Red is always an assault, an insult, a danger cry, shouting Revolution! Robbery! and paradoxically, 'Homage to the King'. It is the taunting flame out of the primal volcano. It is the easiest colour to see. Man saw it first. Orange is an open colour expressing prosperity and plenty, sunbaked universe, and laughter under the sun.
Yellow is the atmosphere of wisdom, reflection and calm. Green is quieter still, rest and content, the emerald ripple of wave and flow. Blue is the colour we love most, its suggestion is the lark's song, hope that soars into the stratosphere. Indigo is tragedy, like red it can stand almost alone, crying to Pluto's intense blackness, to death, and to Faith.
This conjures violet, whose magic is perceived only by keen-eyed men, but it is known by song birds and honey bees. Its wish can only be used by the great colour masters, and it is a safe indication of their mastery. It has been caught best by the Eastern painters, seeing in psychic sunlight [...] for it calls to a colour beyond itself on the scale, a colour that our eyes cannot see, although we know that it is there by the power of its ultraviolet rays. Maybe we shall see this colour some day when we have trained our eyes more precisely. Some eyes even now, looking at a rainbow or prism, can see beyond the violet, a faint trace of fuchsia pink, the indication of the red, the first colour of the rainbow into which the colours flow in their completed cycle. For past the gap we cannot see, the violet flows back into red again. Look at the double rainbow in the stormy sky. Can your eyes see a hint of this unknown colour between the outer bright rainbow and its echo?"
Winifred Nicholson, Liberation of Colour.