To perform a color matching operation for color tones of a repairing part and a part-to-be-repaired of external facing of a concrete structure, on the basis of color samples.

A group of color samples K2, k3, kn and Kn+1 corresponding to respective different color tones of a repairing part 10 is provided. Subsequently, a color tone of a part-to-be-repaired 12 around the repairing part 10 is measured; information 14 on an index value and the like of the color tone of the part-to-be-repaired 12 is grasped; and information 14 on an index value and the like common to or akin to the information 14 on the index value and the like of the color tone of the part-to-be-repaired 12 is selected from the group of color samples K2, k3, kn and Kn+1 so that a coloring agent for the repairing part 10 can be prepared and applied to a surface layer part of the repairing part 10.