Where two colors of similar lightness are placed side by side, they often appear as a light reflected on one side and a shadow on the other along the boundary. These appearances are quite exciting where saturated warm colors are concerned. So, it is called the vibrating boundary. To research such a phenomenon the author tried one experiment with color-line samples. From the experimental results, it is assumed tha the appearances relate to such a contour phenomenon of warm and cold colors as the Mach band in the case of brightness contrast. And the appearances of the "vibrating boundary" are also discussed for the application to designing.