Colour theories have been established in order to allow artists and designers to understand the relationship between colours, and to determine how a chosen colour will react or interact with another. Smart colours - such as leuco dye-based thermochromic inks - have entered into the textile and fashion design world, bringing with them new challenges. This paper stresses the fact that, in order to demonstrate and describe the behaviour of these inks at different temperatures when printed on textiles, a new colour system is required. Four design experiments using thermochromic inks are described and these, along with the discussion presented, highlight the need to develop a thermochromic colour system which can not only visualise and describe the behaviour of these inks in relation to other static pigments and temperatures, but also assist textile designers in making informed decisions during the design process.