An image forming apparatus capable of preventing print setting information of print data transmitted from an external apparatus from being changed into infeasible information. A CPU of the apparatus stores print data transmitted from the external apparatus. The print data includes a content to be printed and print setting information indicating whether the content is to be printed in color or in monochrome. The CPU determines whether or not the content is chromatic. The CPU receives an instruction for changing the print setting information from a user, before starting to print the print data. In a case where the print setting information indicates that the content is to be printed in monochrome, the CPU permits changing the color mode information as for a content determined to be chromatic and restricts changing the color mode information as for a content determined to be not chromatic.