Color to Gray and Back transformation for distributing color digital   images
  The Color to Gray and Back transformation watermarking with a secrete key is considered. Color is embedded into the bit planes of the luminosity component of the YUV color space with the help of a block algorithm that allows using not only the least significant bits. An application of the problem of distributing color digital images from a data base among legitimate users is discussed. The proposed protocol can protect original images from unauthorized copying. 
V.N. Gorbachev, E.M. Kaynarova, I.K. Metelev, E.S. Yakovleva
cs.MM