This thesis develops and explores a constitutive approach to colour vision, which serves as an alternative to the standard experiential view of colour vision operating in the philosophy of colour. The approach seeks to describe the nature or essence of colour vision qua psychological kind. I argue that it is constitutive of colour vision that an organism possesses the ability to achieve colour constancy. An important feature of my account is that colour constancy is characterised as the ability to discriminate differences in surface reflectance properties across changes in illumination conditions. This differs from the standard ‘appearance invariance view’, which characterises colour constancy by appealing to the phenomenology of apparent colour. I consider an important objection to the appearance invariance view posed by the argument from illumination, which might also seem to carry over to the reflectance discrimination view. The objection is based on the claim that in standard cases of colour constancy the phenomenology of apparent colour is partly illumination-dependent. I argue that the reflectance discrimination view is perfectly able to accommodate this point. As a case study in applying the constitutive approach to illuminate the distinctive nature of colour vision, I argue that a vivid feature of our ordinary experience of colour known as categorical perception should be dissociated from our colour vision abilities. Although colour ontology often is not at the forefront of discussion, these constitutive theses support the ontological view of colour known as reflectance physicalism. I critique the argument from colour similarity, which many take to pose the greatest threat to reflectance physicalism. The thrust of the argument is that colours phenomenally appear to stand in similarity relations that do not correlate with the similarities that are evident among reflectance properties. This argument lacks much force, however, as it fails to acknowledge the extreme context sensitivity of similarity and the presentation sensitivity of our knowledge of similarities.