My answer to the first question that is posed in the title of this thesis is that colours are homogeneous, which means each and every colour is only one in number. This means that colours are not heterogeneous, that is, they are not compounds or mixtures. For example: orange is often said to be red and yellow, and grey is often said to be white and black. In other words: orange and grey are both claimed to be heterogeneous. However, my conclusion that colours are homogeneous simply excludes that heterogeneity can be the case. My answer to the second question is that colours are two-dimensional, which means that colours stretch out in length and breadth, but not in depth. This conclusion gainsays naïve realistic conceptions about colours, for example that they can be objects like a piece of blue cobalt, or that there can be voluminous coloured light beams passing in three-dimensional space from a light source and, when they hit objects, mix with their colours. For example, one use to say that yellow and purple beams colour a landscape at sunset. The conclusion on two-dimensionality also gainsays the more sophisticated theory of identification of colours with brain events. That is, colours cannot be identified with brain events because the latter are threedimensional while the former are two-dimensional. These two conclusions are drawn from three general propositions, which I call Basic Suppositions. The first says there is concomitance between colours and their extensions. This means that any colour has a certain extension and that this extension cannot be separated from the colour itself. It follows that colours are homogeneous because if heterogeneous, like the contention on orange, the implication will be orange is twice its own extension, and this contradicts the first basic supposition. The second says that colours can only relate beside each other. This basic supposition gainsays naïve realistic conceptions which include that colours might exist behind each other and have different directions in three-dimensional space. The third says that only colours can limit colours, which means there can be no empty space or “clear air” between any two colours, i.e., it cannot be a blank or a gap between them, which is not a colour. In addition, my inquiry results in two other basic suppositions, namely that colours might be identical notwithstanding difference in figure, size or position, and that two or more different colours cannot be identical with one and the same colour. All these propositions will be clarified and defended in the discussion to follow.