Yoko Ono is the creator of an art book called Grapefruit. The book is hard to describe.
It is a series of recipes for art pieces to be executed. Also known as instructional poems (which I find to be a delightful synonym for the word recipe).
My English teacher in school was passionate about teaching us the meaning of poetry. He read many famous poems out loud to the class. However, one day he read a recipe for stew pulled from an actual cookbook, i.e. the ingredients list and instructions for cooking. He argued the case that it was a poem, that it was a series of words carefully composed to convey a meaning. At that moment I felt my perceptions slide sideways and I was captivated by the concept. The recipe itself was its own piece, regardless of the product it was supposed to cause to come into existence. I suddenly believed that an idea can be the whole work of art.
In this way, it is clear that the real art piece is meant to occur inside the reader's (or viewer's) mind. It is also a way of using a language as a medium for art, yet without falling into the category of writing or poetry. These are structured individual pieces in a series.
'Colour Piece' is one that feels like an actual poem. It is one of my favourites in the book, if only for the clever twist of still being formulaic, like the others pieces - yet vague. "Have absent thoughts for a long time" is not a definitive amount of time.