Throughout her philosophical writing, Margaret Cavendish is clear in stating that colours are real; they are not mere mind-dependent qualities that exist only in the mind of perceivers. This puts her at odds with other seventeenthcentury thinkers such as Galileo and Descartes who endorsed what would come to be known as the ‘primary-secondary quality distinction’. Cavendish’s argument for this view is premised on two claims. First, that colourless objects are inconceivable. Second, that if an object is inconceivable then it could not possibly exist in nature. My aim in this paper is to explain why Cavendish accepts both premises of this argument. However, the repercussions of this paper go much further than just explaining why she thinks colourless objects cannot exist in nature and the upshots are twofold. First, it provides new insights into the fundamental role that perception plays in Cavendish’s metaphysics. While Cavendish’s view that all of nature perceives is well-established, I show that Cavendish is also committed to the view that all of nature is perceivable. Second, it provides the first in-depth discussion of Cavendish’s modal epistemology and her reasons for thinking that inconceivability entails impossibility (in nature).