We have developed a reversible method to convert color graphics and pictures to gray images. The method is based on mapping colors to low-visibility high-frequency textures that are applied onto the gray image. After receiving a monochrome textured image, the decoder can identify the textures and recover the color information. More specifically, the image is textured by carrying a sub-band (wavelet) transform and replacing band-pass sub-bands by the chrominance signals. The low-pass sub-band is the same as that of the luminance signal. The decoder performs a wavelet transform on the received gray image and recovers the chrominance channels. Registration problems are discussed and examples are presented.
Karen M. Braun, Ricardo L. de Queiroz, "Color to Gray and Back: Color Embedding Into Textured Gray Images" in Proc. IS&T 13th Color and Imaging Conf., 2005, pp 120 - 124, https://doi.org/10.2352/CIC.2005.13.1.art00023