1. A method of reproducing a coloured original in which the original is scanned to obtain signals representing three colour-component densities of successively scanned picture elements of the original and in which said signals are corrected before being used to control the treatment of an output surface, said method comprising, prior to the scanning of the original, the steps of deriving, for each of a number of sets of three input digital colour-component values fewer in number than the sets of possible input digital colour-component values available from the scanner, a set of four corrected colour component values in digital form, one for each of the three colour components and one for black, and loading the sets of four corrected colour-component digital values into storage locations of a three-dimensional store, said store being represented as a cubic matrix consisting of adjacent unit interpolation cubes, each unit interpolation cube being formed by an addressed point and the seven neighbouring points which would be addressed by incrementing by one "coarse" unit, each colour-component density signal value singly and in all combinations, so that the locations of the corners of the cubes represent the three-dimensional digital colour-component values, the corresponding sets of four corrected colour-component values being stored at those locations; and thereafter addressing locations of the store by means of the succession of sets of three colour-component signals derived from the scanning of the original to obtain from the store a corresponding succession of corrected digital values, obtaining values intermediate the sets of digital values stored for unit interpolation cubes of said matrix by on-line interpolation, controlling the treatment of the output surface in accordance with the four values obtained from the interpolation, in which each of the four values for a point within a unit interpolation cube of the matrix is obtained by first determining in which of three pyramids, which together make up the unit interpolation cube and have as a common edge the "neutral axis" of the unit interpolation cube, said point is contained, and deriving a value for said point by interpolation within the pyramid, whereby each of the values for said point is the sum of predetermined functions of the values for the vertices of the pyramid, said functions also varying with the distances of said point from faces of the pyramid.