Already in the late 1940s, Rupprecht Geiger attained an abstract notion of pictures, regarding colour as the main means of expression in painting. Based on tonal value studies and experiments on the energetic qualities of colour values, he developed compositions of geometric colour fields. He preferred simple geometric forms, such as rectangle, square, oval and circle, and vibrant colours, sometimes applying a single colour, sometimes merging several. In their lyrical quality, his colour compositions distinguish themselves from a constructive approach.
Together with Willi Baumeister, Rolf Cavael, Gerhard Fietz, Willy Hempel, Brigitte Matschinsky-Denninghoff and Fritz Winter, Geiger founded the 'Gruppe der Gegenstandslosen' in 1949, renamed to 'ZEN 49' later that same year. In the years 1959 to 1977, Geiger took part in documenta in Kassel several times. In 1965, he was appointed professor at the Düsseldorf Art Academy (until 1976); a solo exhibition of his works was held that year at the Von der Heydt-Museum in Wuppertal, and another one in 1967 by the Kestner-Gesellschaft in Hannover. On occasion of his 100th birthday, the Städtische Galerie im Lenbachhaus, Munich, devoted a retrospective to his work in 2007.