A new principle is proposed to pseudocolor-code the differential interference in the grating-diffraction interferometry with extended white light illumination by using dispersion effect of a prism. The phase gradients of the tested object are shown by coding colors monotonically with a determinating period. The conditions to yield such coding colors as spectral colors, mixing colors of high purity or fullcolors are discussed. Possible profiles of gratings are then given to obtain an exact differentiation. So, real and effeotive oo]or coding of in-terferomeirioal fringes is realised for the first time.