The raw, early Earth had plenty of colour, but that was nothing compared with what was going to come next. That canvas was about to be painted with a vast new palette - and the source of those colours was life. Green is the colour of the natural world and yet it's the one colour that plants have evolved not to use. The huge diversity of human skin tones tells the story of how humanity spread and ultimately conquered the planet. But the true masters of colour turn out to be some of the smallest and most elusive. Helen Czerski travels to the Great Smokey Mountains of Tennessee during the one week in the year when fireflies light up the night sky with their colourful mating display. And she reveals the marine animals that hide from the world by changing the colour of their skin.