In painting, Post-impressionism consists of a collection of works which appeared in the two last decades of 19th century in Europe. Although these artists never formed an official group, the purpose of their work was to amend impressionism's shortcomings. The most important quality that is observed in post-impressionist paintings which this research notes, is the abundance of variety of color spectrums gathered into a single painting. Multiplicity in this research, is an idea loaned from French philosopher Gilles Deleuze and is a method to describe the plurality of all beings and to demonstrate the new ways of an ever-evolving nature. This research tries to describe how the multiplicity of color appears in post-impressionism and answers the question of how multiplicity of color has influenced different domains of expression in the art of painting and what the results are. The multiplicity of color is an unitarianist answer to the historical dualities of Europe painting such as "art & nature" or "objectivity & mentality", and post-impressionism through that makes these contrasts to unity. The multiplicity of color makes "expression" the main purpose of painting and sets it in contrast of pure "representation". For this we analysed paintings of artists from the last two decades of 19th century who mainly worked in France, specifically George Surat, Vincent Van Gogh, Paul Gauguin and Paul Cezanne. This research concludes that form and color in paintings of Cezanne and Von Gogh, surface and perspective in Surat's works, and aesthetics and personal impression of nature in Gauguin's works reach unity. Therefore one of the other reasons of using multiplicity of color in post-impressionism was filling the historical gap between artist and the universe in impressionism and the requirement of creating balance and unity between historical dualities of European's painting and the domains of its effects include the orders of form, color, aesthetics and surface and perspective. This research also concluded that extracting of figure from figurative order and rediscovery of portrait in paintings of Von Gogh and Gauguin and extraction of nature's figure in Cezanne's works are the results of multiplicity of color in the field of expression in post-impressionism. Finally we concluded that multiplicity of color in post-impressionism paintings is the result of utilizing the expression machines, such as dot-color machine in Surat's paintings, line-color in Von Gogh's paintings, stain-color in Cezanne's paintings and surface-color in Gauguin's paintings .These machines take the nature as the entrance and convert it to dot, line, stain and surface and the factor of their connections is the color modulation plan which the expression machine works on it and merge its functions with each other. The modulation plan is a non-transcendental field which based on it the machines can connect to each other by the multiple directions and create bigger machines and wider surfaces. The relations on modulation plan that lead to connecting machines together are the familiarity relationships, contradiction and complementary. The method of research is descriptive-analytical, the method of analysis is qualitative and method of data gathering is library research.