This thesis examines how and when infants learn colour words, and how the knowledge of colour words affects their comprehension of colour categories. Over the course of seven experimental chapters, the ability of infants and toddlers to learn colour words, use colour words to process colours, and the role that colour words play in learning to perceive colour are all assessed. Chapters 2 and 3 assess claims that colour words are learned late using parental report and eye-tracking methods, finding that colour words are learned as early as 19 months. In contrast to this, Chapter 4 demonstrates that toddlers do not learn to modify colour words as dark or light until much later. Chapter 5 demonstrates that colour words are a crucial component for processing the colours of objects, showing that infants do not look to a colour-matched object unless they comprehend the colour word. Chapters 6 and 7 employ novel paradigms to explore categorical processing of colour, finding that infants have a preference for within-category colours, but that this has no effect on their attention to dynamic coloured stimuli. In Chapter 8, a prototype for an infant colour vision test is shown, demonstrating that the second year of life is crucial for development of visual closure. The generalisability of these results to infant perception and word learning is also discussed.