Basically this guy, Anish Kapoor, had made a deal with NanoSystem, the people producing Vantablack, or the blackest black paint pigment in the world, so that he would have exclusive rights to use it for art purposes.
This naturally upset a great number of people in the art world, to say the least, as many other people were excited to experiment with using the Vantablack pigment in their art. Therefore, as an act of retaliation, artist Stuart Semple released PINK, the world’s pink pigment under the stipulation that anyone in the world could buy it, except for Anish Kapoor. At the time of purchase, the purchaser has to agree to a legal statement saying that they are not Anish Kapoor and will not willing or knowingly give the pigment to Anish Kapoor.
By this image, it indicates that someone violated that agreement and provided the pigment to Kapoor. Thus why Semple is pursing legal action. I hope this helps clear some things up.