When Vladimir Umanets entered the Tate Modern on October 7, 2012 and defaced Rothko's Black on Maroon, he was operating, not as an artist or a vandal, but as a Yellowist. Yellowism is neither art nor anti-art but is instead a supposedly new cultural element that exists for its own sake and is about nothing but the color yellow. It might be tempting to write Yellowism and the Rothko defacement off as a mere prank or as pseudo-intellectual fraud, but I argue that, intentionally or not, the Yellowists have raised issues salient to those invested in both the ontology of art and social ontology more generally. In particular, their actions highlight issues pertaining to the relationship between stipulation and ontology. I explore these issues in this paper.