wiki: Although Klein had painted monochromes as early as 1949, and held the first private exhibition of this work in 1950, his first public showing was the publication of the Artist's book Yves: Peintures in November 1954. Parodying a traditional catalogue, the book featured a series of intense monochromes linked to various cities he had lived in during the previous years. Yves: Peintures anticipated his first two shows of oil paintings, at the Club des Solitaires, Paris, October 1955 and Yves: Proposition monochromes at Gallery Colette Allendy, February 1956. Public responses to these shows, which displayed orange, yellow, red, pink and blue monochromes, deeply disappointed Klein, as people went from painting to painting, linking them together as a sort of mosaic.
From the reactions of the audience, [Klein] realized that...viewers thought his various, uniformly colored canvases amounted to a new kind of bright, abstract interior decoration. Shocked at this misunderstanding, Klein knew a further and decisive step in the direction of monochrome art would have to be taken...From that time onwards he would concentrate on one single, primary color alone: blue.
The next exhibition, 'Proposte Monochrome, Epoca Blu' (Proposition Monochrome; Blue Epoch) at the Gallery Apollinaire, Milan, (January 1957), featured 11 identical blue canvases, using ultramarine pigment suspended in a synthetic resin 'Rhodopas,' described by Klein as "The Medium." Discovered with the help of Edouard Adam, a Parisian paint dealer, the optical effect retained the brilliance of the pigment which, when suspended in linseed oil, tended to become dull. Klein later patented this recipe to maintain the "authenticity of the pure idea." This colour, reminiscent of the lapis lazuli used to paint the Madonna's robes in medieval paintings, was to become famous as International Klein Blue (IKB). The paintings were attached to poles placed 20 cm away from the walls to increase their spatial ambiguities.
The show was a critical and commercial success, traveling to Paris, Düsseldorf and London. The Parisian exhibition, at the Iris Clert Gallery in May 1957, became a seminal happening. To mark the opening, 1001 blue balloons were released and blue postcards were sent out using IKB stamps that Klein had bribed the postal service to accept as legitimate. Concurrently, an exhibition of tubs of blue pigment and fire paintings was held at Gallery Collette Allendy.