An image processing apparatus processes multivalued color image data that includes a plurality of color components. The apparatus includes an adding unit to add per-color-component quantization error values from a neighboring pixel to respective ones of a plurality of color components included in a pixel of interest, and a quantization unit to correlate a sum of the values of each of the color components to which the error value has been added by the adding unit to a sum of values after conversion, and deciding the value of each color component after conversion upon distributing the sum of pixel values after the conversion in accordance with the ratio of each of the color components before conversion. In addition, a coefficient decision unit decides diffusion coefficients, which diffuse quantization error to neighboring pixels, based upon the value of each of the plurality of color components included in the pixel of interest.