Mean Value

Grey is in the middle between light and shade, a mixture of all colours merged into a single one. In front of a grey wall all other colours appear with the same power. Through its neutrality it gives the others the equal conditions of light value and colour expression so several galleries prefer grey walls.
A white background gets into concurrence with the brighter colours [yellow, orange, lime green] and lets them appear as a darker colour. On the other hand black turns colours seeming to be brighter but puts darker tones into trouble to evolve their power. It is exciting, when prismatic colours are used instead like it was the case a the time before 1920.