While in Paris, Kelly had continued to paint the figure but by May 1949, he made his first abstract paintings.Observing how light dispersed on the surface of water, he painted Seine (1950), made of black and white rectangles arranged by chance. In 1951 he started a series of eight collages titled Spectrum Colors Arranged by Chance I to VIII. He created it by using numbered slips of paper; each referred to a colour, one of eighteen different hues to be placed on a grid 40 inches by 40 inches. Each of the eight collages employed a different process.Kelly's discovery in 1952 of Monet's late work infused him with a new freedom of painterly expression: he began working in extremely large formats and explored the concepts of seriality and monochrome paintings. As a painter he worked from then on in an exclusively abstract mode. By the late 1950s, his painting stressed shape and planar masses (often assuming non-rectilinear formats). His work of this period also provided a useful bridge from the vanguard American geometric abstraction of the 1930s and early 1940s to the minimalism and reductive art of the mid-1960s and 1970s. Kelly's relief painting Blue Tablet (1962), for example, was included in the seminal 1963 exhibition, Toward a New Abstraction, at the Jewish Museum.
In the 1970s he added curved shapes to his repertoire.Green White (1968) marks the debut appearance of the triangle in Kelly’s oeuvre, a shape that reoccurs throughout his career; the painting is composed of two distinct, shaped monochromatic canvases, which are installed on top of each other: a large-scale, inverted, green trapezoid is positioned vertically above a smaller white triangle, forming a new geometric composition.After leaving New York City for Spencertown in 1970, he rented a former theater in the nearby town of Chatham, allowing to work in a studio more spacious than any he had previously occupied. After working there for a year, Kelly embarked on a series of 14 paintings that would become the Chatham Series. Each work takes the form of an inverted ell, and is made of two joined canvases, each canvas a monochrome of a different color. The works vary in proportion and palette from one to the next; careful attention was paid to the size of each panel and the color selected in order to achieve balance and contrast between the two
In later paintings, Kelly distilled his palette and introduced new forms. In each work, he started with a rectangular canvas which he carefully painted with many coats of white paint; a shaped canvas, mostly painted black, is placed on top.
In reference to his own work Kelly said in an interview in 1996: "I think what we all want from art is a sense of fixity, a sense of opposing the chaos of daily living. This an illusion, of course. Canvas rots. Paint changes color. But you keep trying to freeze the world as if you could make it last forever. In a sense, what I've tried to capture is the reality of flux, to keep art an open, incomplete situation, to get at the rapture of seeing."
Kelly commented "I realized I didn't want to compose pictures … I wanted to find them. I felt that my vision was choosing things out there in the world and presenting them. To me the investigation of perception was of the greatest interest. There was so much to see, and it all looked fantastic to me...Wikipedia