1. A color adjusting method of adjusting colors of a color image, comprising: adjusting a predetermined adjustment range of a color of the color image using a processor by converting a value of a parameter representing a straight line passing through a useful adjustment range of a color to be adjusted in a color space representing the color image, the predetermined adjustment range including and being larger than the useful adjustment range such that a lowest value of the useful adjustment range becomes a non-zero value after the adjustment; and outputting an adjusted color to an output device, wherein luminance of the color image is adjusted by a smooth curvature reference conversion function, and chromaticity of the color image is adjusted by a non-curvature reference conversion function.