The paper considers major approaches of digital images tone approximation, which consists in reduction of image palette size, and their classification. The main drawbacks and advantages of each approximation method are discussed. Also discriebed hybrid algorithm of monochrome images tone approximation that developed and investigated by author.The hybrid algorithm consist in combination of heuristic and deterministic approach. The heuristic stage is based on evolutionarily-genetic algorithm. The main goal of heuristic stage is reduction the area of searching. Such a role for heuristic stage defined according to his fast computational time. The goal of deterministic algorithm consist in finding a nearest extreme for the result that obtained by previous algorithm. The results of comparative investigations are discussed.

Keywords: tone approximation, optimization, monochrome images, evolutionary-genetic algorithm, k-means algorithm, color quantization, median cut, images compression, pattern recognition, images approximation, images processing