The first colour charts were unsystematic. They were based directly on commercial colour samples. They were still related to Pop Art. In the canvases that followed, the colours were chosen arbitrarily and drawn by chance. Then, 180 tones were mixed according to a given system and drawn by chance to make four variations of 180 tones. But after that the number 180 seemed too arbitrary to me, so I developed a system based on a number of rigorously defined tones and proportions.

The smudging makes the paintings a bit more complete. When they're not blurred, so many details seem wrong, and the whole thing is wrong too. Then smudging can help make the painting invincible, surreal, more enigmatic - that's how easy it is.

The photograph is the most perfect picture. It does not change; it is absolute, and therefore autonomous, unconditional, devoid of style. Both in its way of informing, and in what it informs of, it is my source.

In 1981, I think, for the Kunsthalle in Düsseldorf. Before that I designed a mirror room for Kasper König's Westkunst show, but it was never built. All that exists is the design - four mirrors for one room.

I don't believe in the reality of painting, so I use different styles like clothes: it's a way to disguise myself.

I was a student, and as such you generally rely on prior models of how to make art, but these were not satisfying. Then I discovered in photos what had been missing in paintings; namely that they make a terrific variety of statements and have great substance. That is what I wanted to convey to paintings and apply to it.

Turning to the colour-classification methodology: The starting point are the four pure colours red, yellow, green and blue; their in-between shades and scales of brightness result in colour schemes containing 16, 64, 256 and 1,024 shades. More colours would be pointless because it wouldn't be possible to distinguish between them clearly.