1. A position adjustment method for projection images for adjusting positions of two projection images projected from two of plural projectors contained in a multi-projection display onto a projection surface such that the two projection images have an overlapping area by using two adjustment images one of which has a first pattern and the other of which has a second pattern, comprising: setting pixel values of a first color and pixel values of a second color such that a predetermined characteristic is exhibited in the overlapping area when the two adjustment images are projected at appropriately correlated positions, producing two adjustment image data, corresponding to the two adjustment images, in which the first color and the second color are allocated to the first pattern and the second pattern such that the first color of the first pattern overlaps with the second color of the second pattern and such that the second color of the first pattern overlaps with the first color of the second pattern when the two adjustment images are projected at appropriately correlated positions, and giving the adjustment image data to the two projectors; calculating evaluation values associated with the predetermined characteristic based on image data obtained by taking an image of the projection surface on which the two adjustment images corresponding to the two adjustment image data are projected from the two projectors; and performing position adjustment for the two projection images based on the evaluation values, wherein at least one of the above steps is performed by a control unit.