Multi-primary color display devices, which havemore than four primaries, require more than two sets of control signals to reproduce specific tristimulus values. However, rapid changes in the control signals, even when the reproduced tristimulus values change smoothly, is considered to cause degradation of image quality. Smooth multiprimary color decomposition, which means that the control signals change smoothly when the tristimulus changes smoothly, is required. To meet this requirement, we propose a new method for smooth multi-primary color decomposition based on the spherical average of signal values.