According to a hue conversion, hue angles of C, M, and Y are always set to hue angles HC, HM, and HY of C, M, and Y of a printer, hue angles of R and G are always set to hue angles HR and HG of R and G of a monitor, and the hue angle of B is always set to a value HB desired by a user. The gradation from black through a full color to white is made linear in each color of R, G, B, C, M, and Y according to the hue conversion. The user sets the hue angle HB for B. Therefore, the user can obtain blue color B which provides the user's favorite hue and gradation. Every hue can be reproduced excellently and gradations can be reproduced without color shifts.