Ralph Waldo Emerson defined knowledge as the knowing that we cannot know. What colour is the sky? We all know that, when there are no clouds, the sky has a bluish colour, which we refer to as “sky blue”. This is because it is a mixture of blue, indigo and violet colours. But did you know that the sky really has no colour? In fact, outside of the earth’s atmosphere, the sun is white and the sky is dark; it is colourless!