An improved method of operating a color scanning system of the type that incorporates a monochrome image sensor having at least one photosensitive area that is sequentially exposed to individual colors of a scene and to a dithering of the scene with respect to the image sensor photosensitive area is described along with an improved color filter mask for the at least one photosensitive area. The blue filter mask is formed over a portion the photosensitive area, for example, an L-shaped portion, that covers approximately 75% of the photosensitive area. The method of operation is to first image and dither a red filtered version of the scene onto the photosensitive area to generatr a red image signal. Next a green filtered version of the scene is imaged and dithered onto the photosensitive area to generate a green image signal, and lastly a blue filtered version of the scene is imaged onto the photosensitive area without dithering to generate a blue image signal. The photosensitive area generates an electrical signal from it's full area in this mode, because the blue masking material is transparent to the blue image.