In which we examine the dioptrical colours arising when light passes through media of varying transparency, revealing how subtle changes in density of a colourless medium can produce vibrant yellows, reds, and blues as observed in atmospheric phenomena, fluids, and solids. Through careful observation and experiment, these physical colours are shown to depend not only on light and the medium but also on the interplay with darkness, offering a nuanced understanding of the colouring of nature and art.