This paper focused on making use of color change to incarnate hierarchy of floodlighting of the building facade in landscape lighting. Color lapse by halo retreat is a good method to reach this goal. The projection light source was chosen on the principle of building exterior color coordination. According to the computing method for chroma coordinates of object color, the magnitude of three stimulus values after reflection by external wall surface and the chroma coordinates were calculated. The purpose of this paper is to take account of the influence of difference between projecting light and reflecting light on external wall surface. At the same time, an approximating computing method is recommended, that is, first figure out the chroma coordinates of two points--the top and bottom points in vertical line of the building and then readjust the location of casting light lamp and lighting fitting or renewal the light source. Then let the chroma coordinate of ever point on the vertical line settle in or be near the line connecting the two points of chroma coordinates on CIE 1931 Chroma chart. It would yield good results due to the color lapse by halo retreat.