1. An image generation apparatus comprising: a memory; and a processor programmed to generate image data of an evaluation image having a first reference color, a second reference color, a first evaluation color, and a second evaluation color, the first reference color and the second reference color being determined using a relationship in which the first reference color and the second reference color are paired in a predetermined color space, the first evaluation color and the second evaluation color being colors defined between the first reference color and the second reference color in the predetermined color space relative to a color gamut, the first evaluation color being determined with respect to the first reference color, the second evaluation color being determined with respect to the second reference color, the first and second reference colors being closer than the first and second evaluation colors to outer edges of the color gamut, the evaluation image being generated by selecting four different colors for the first reference color, the second reference color, the first evaluation color, and the second evaluation color, the evaluation image being formed such that the first evaluation color is positioned inside the first reference color and not inside the second reference color and such that the second evaluation color is positioned inside the second reference color and not inside the first reference color, an image having the first evaluation color being surrounded by and in contact with an image having the first reference color, and an image having the second evaluation color being surrounded by and in contact with an image having the second reference color; output the image data to a display device that is caused to display the evaluation image, the display device having a lamp that displays the entire evaluation image; adjust at least one of the first and second evaluation colors until the image having the first evaluation color looks the same as the image having the second evaluation color in terms of brightness or tint in a case where the evaluation image is displayed on the display device; retain image data of the first evaluation color and image data of the second evaluation color of the evaluation image obtained when the image having the first evaluation color looks the same as the image having the second evaluation color in terms of brightness or tint in the displayed evaluation image; stop display of the evaluation image; re-display the same evaluation image including the retained image data of the first evaluation color and the retained image data of the second evaluation color after the display device has been used for a predetermined period; and determine whether the lamp of the display device has deteriorated based on whether the image having the first evaluation color still looks the same as the image having the second evaluation color in terms of brightness or tint in the re-displayed evaluation image.