Complex sentence: The original chair was not painted in the familiar De Stijl pallette of primary colours, black, grey and white but was later changed to mimic the neo-plastic paintings of Piet Mondrian after Rietveld came into contact with the artist's work in 1918.
Split into simpler sentences: The original chair was not painted in the familiar De Stijl pallette of primary colours, black, grey and white. However, it was later changed to resemble the paintings of Piet Mondrian when Rietveld came into contact with this artist's work in 1918.