Dichromats recognize colors using two out of three cone cells; L, M, and S. In order to extend the ability of dichromats to recognize the color difference, the authors propose a method to expand the color difference when observed by dichromats. Most methods analyze the color in color space, while their method analyzes the color in image space. Namely, they analyze the color between the neighboring pixels not in intensity space but in chromaticity space and form a Poisson equation. Solving the Poisson equation results in an image for dichromats whose relative color difference between neighboring pixels is as close as the image observed by trichromats.