The leaves of the trees in the forest, when there is no one around to see them, are colourless – as is everything else. Why? Because there is no such 'thing' as colour; it only exists when there are animals with certain kinds of perceptual systems to perceive it. And also, because everyone's perceptual system is unique, and so therefore are their perceptions, no two animals, human or otherwise, ever see exactly the same colours.