Image processor and image processing method having correcting unit performing correction based on correction parameters

When printing an image using a plurality of inks, color unevenness caused by variations in ejection characteristics among nozzles is corrected at suitable timings in accordance with change in the color unevenness over time, and favorable image output without noticeably color unevenness is maintained. For this purpose, parameters are prepared, for colors formed by combinations of at least two colors of ink, the parameters being determined so as to reduce differences in coloration on a print medium caused by individual variations in the ejection characteristics of a plurality of nozzles. When printing, a first color signal included in individual pixels is corrected for a second color signal by using the parameters. Information regarding the ejection volume characteristics of a plurality of nozzles is acquired as appropriate, and by estimating changes in coloration from this information, suitable timings for overwriting such parameters are determined.