A level compression and/or gradation conversion of a color video signal are performed without causing any change in the hue of a picture-represented by the color video signal. The knee compression and/or gradation conversion are carried out by multiplying' red, green and blue primary color signals by a luminance gain, while the hue and saturation remain unaffected. If the level of any of the primary color signals still exceeds a predetermined threshold level, the saturation conversion is executed by using the saturation gain and the lumizlanc.e signal supplied from a controller. In the saturation conversion operation, the maximum level of at least one of the red, green and blue primary color signals is made coincident with the predetermined threshold level while the hue and luminance of the picture represented by the color video signal are maintained unchanged.