Disclosed are a method and apparatus for forming a color transform lookup table which helps output a smooth image having no pseudo contour and excellent in granularity. On the basis of a previously formed table, a color space is expressed by a basic triangle of white-primary color-black, and output signal values corresponding to lattice points in this basic triangle are calculated by linear interpolation. Black ink and cyan ink are replaced by setting a straight line AB, as a boundary line, connecting two points immediately before black ink begins to be introduced on two sides of the basic triangle.