A 3-D lookup table (LUT) approach has been developed for color separation for color inkjet printers to better control the color separation and ink limit. This method starts from the color separation for critical points, followed by 1-D interpolation to determine the color separation for grid points in critical lines, followed by 2-D interpolations to determine the color separation for grid points in critical planes, followed by 3-D interpolation to determine the color separation for the remaining grid points in the 3-D LUT. To control the spreading of K and high-density ink to some regions, such as the highlight region and the flesh tone region, we start from controlling the ink propagation in line interpolation, which is a fairly easy step, then control the ink propagation in planes, and finally control the ink propagation in 3-D interpolation. With this process, a 3-D LUT for the conversion from a printer RGB space or a virtual CMY space to an n-colorant space is built to maximize the printer gamut and to minimize grain. This paper describes the details of special interpolations to fill the grids in a 3-D color separation LUT, which includes controlling ink propagation in grids on selected lines, followed by special interpolations to minimize grain in plane-interpolation, followed by special interpolations to fill the remaining grids in the 3-D LUT.