1. An image processing apparatus having a reading unit for reading an original, wherein the image processing apparatus comprises: a converting unit configured to convert a color image into a monochrome image for output of the monochrome image on a recording medium, wherein in a case where the color image is formed by reading an original with the reading unit, the converting unit converts the color image into the monochrome image using a first color-monochrome conversion method for executing conversion in such a manner that security information included in the color image is difficult to be discriminated by human eyes, and wherein in a case where the color image is formed from a job input from an external apparatus, the converting unit converts the color image into the monochrome image using a second color-monochrome conversion method selected from among a plurality of color-monochrome conversion methods via an operation unit provided in the external apparatus or in the image processing apparatus; and an output unit configured to output the monochrome image converted by the converting unit on the recording medium.