A method for discriminating a color of an object image. Whether unit data showing a color of a part of the object image corresponds to any position on a two-dimensional color plane is discriminated. A distribution value showing a width of a distribution range of the unit data in the color plane is acquired based on the discrimination result. Whether the object image corresponds to a full color image having mixed color expression, a specific color image not having mixed color expression or an achromatic color image is discriminated based on the distribution value.