Yves Klein is best known for his trademark ultramarine pigment, which he patented as International Klein Blue in 1961. “Blue…is beyond dimensions, whereas the other colors are not,” he said. “All colors arouse specific ideas, while blue suggests at most the sea and the sky; and they, after all, are in actual, visible nature what is most abstract.” Starting in the mid-1950s, Klein made retinal blue monochromes (which would prove cornerstones of Minimalism) and the pigment would also feature prominently in his Anthropometry paintings, for which Klein smeared nude women with blue pigment and used them as human brushes on canvas, sometimes in elaborate public performances. Klein's work anticipated Conceptual art, Performance art, and environmental art, as in his selling of portions of empty space to collectors. For The Void (1958), he presented an empty gallery as an artwork, wearing a white tie and tails to show visitors around the blank walls.