We have developed a model to calculate chromaticity difference from a surface defined from the MacAdam ellipses in CIE 1931 (x,y)-chromaticity diagram. The idea is to tranform the MacAdam ellipses to circles on a surface. The height of the surface is obtained by projecting a circle of constant radius such that the projection is equal to the ellipse on the the (x,y)-plane. The chromaticity difference is calculated from the surface by a method based on the Weighted Distance Transform On Curved Space (WDTOCS).