BLACK is now technically classified as a colour, being a terminal member of the so-called achromatic series of colours, ranging through a scale of greys to white. From the psychological standpoint black is as positive a phenomenon as any other colour, and cannot be identified with the absence of visual consciousness. It is also a mistake to regard black as an accompaniment of non stimulation of the visual sense organs, since the normal result of this condition is a variable grey. In order to secure a true black either simultaneous or successive contrast is required, involving concomitant excitation of an outlying retinal area or prior excita tion of the same area. respectively.
See Psychological Review, vol. xxxii., p. 248, and vol. xxxi., p. 498.