1. A method executed by a processor of the image generation system, the method comprising: obtaining an image of a first area included in a first image and an image of a second area included in a second image; calculating a first conversion parameter for converting the image of the first area such that color information regarding the image of the first area becomes similar to color information regarding the image of the second area; converting the first image using the first conversion parameter; and generating a third image as a training image used for machine learning for image recognition by combining the converted first image and the second image with each other.