PURPOSE: To provide an improved color picture converting method by which the color (RGB) display of picture elements of a picture can be converted into luminance/color difference (YST) display.

CONSTITUTION: The luminance parameter Y at every picture element of the array (main array) picture elements of a picture is found by computing RGB color parameters and two color difference parameters S and T are computed on all picture element of the picture. The color difference parameters are stored only for the position of a prescribed sub-array (second array) and the color difference parameter stored at the sub-array position receive contributions from the color difference parameters at adjacent positions. When the picture is reproduced and displayed, the color parameters at RGB at each picture element position of the picture are (mostly) reconstituted by combing the luminance parameters at all picture element positions with the color difference parameters related to nearby sub-array positions.