During World War II Nieuwenhuys painted almost only clowns, nudes and couples making love. Just after the war he started painting fantasy animals like aggressive cocks, cats and bulls. He and his brother Constant had many arguments about his paintings. During the war Constant himself painted only Catholic scenes like Pietà and Maria portraits or still lifes and thought that Nieuwenhuys chose his subjects too lightly.
In 1964 he said in an interview: "The group was not founded as an exposition group but as a group of activists. We wanted to put an effort into that, to fight against the softness in art at the time and using our imagination to change that. Regrettably I must ascertain that many of the Experimentalists of that day also became esthetics. Everything official stops to be combatant. I think that work of some of them look very dormant these days".
Nieuwenhuys later concentrated only on his work. His paintings became more and more liberated and he experimented with different materials like fluorescent paint and everything he could get his hands on. Everything could become a painting.
In an interview from 1964 he said: "I start with my material and my color. With that I express myself. From the material I come to my subject and that is maybe contrary to what painters did in earlier days. I paint the way I write, the way I laugh. That is why I paint differently all the time, because my moods change. That’s the way I feel."
From the same interview: "I wish, if they see my work later on, that they can see the twentieth century. The artist must give his time a suit. And it doesn’t matter if he is an architect, poet or painter. In abstract painting I miss the beat of this time, the rudeness. We, the people of today we are living with the fear of an atom bomb. The abstracts are building only a superficial world for you."
He died in Amsterdam in 1986.Wikipedia