Despite the fact about colour, that it is one of the most obvious and conspicuous features of the world, there is a vast number of different theories about colour, theories which seem to be proliferating rather than decreasing. How is it possible that there can be so much disagreement about what colours are? Is it possible that these different theorists are not talking about the same thing? Could it be that more than one of them is right? Indeed some theorists, e.g. Leo M. Hurvich, D. L. McAdam and K. Nassau, say that the term ‘colour’ is used to identify a range of different properties, e.g. pigments, properties of light, and sensations. Such a view has its attractions, but it raises the question of what it is that unites these various concepts – what is it that would make them all concepts of colour? What is it that justifies using the same terms, ‘yellow’, ‘blue’, ‘pink’, mauve’, and so on? This paper aims to address this question, arguing that its answer supports the conclusion that the best theory of colour is a form of anti-realism: the Illusory theory of colours. There are two parts to this thesis, one negative, the other positive. The negative part is that there are no colours, as they are ordinarily conceived. The positive part is that, nevertheless, the world is such that ‘it is as if there are such colours’. Such a theory has important implications. One is that it doesn’t fall neatly into the usual taxonomy of philosophical theories. In particular, it does not deserve the label ‘eliminativist’. Another is that it allows some space for the views expressed by Hurvich, McAdam and Nassau, but not quite in the sense that they intend.