1. An image processing apparatus comprising: a patch forming unit configured to form a plurality of mixed-color patches by superposing first-type single color patches of a first color of a plurality of colors, arranged in an ascending order of gradation values, with a same number of second-type single color patches of a second color of the plurality of colors, arranged in a descending order of the gradation values; a color measuring unit configured to perform at least one color measurement of the mixed-color patches; an estimating unit configured to estimate color values of single colors present in the mixed-color patches based on the at least one color measurement obtained by the color measuring unit; and a correction table setting unit configured to set a correction table to correct the estimated color values of the single colors to target color values.