Mercury is usually golden, Venus turquoise. Earth, though itself brown, is blessedly called the blue planet or green planet. Mars, red. Jupiter is big and yellow, Saturn big and hula-hooped. Neptune and Uranus are traditionally greenish-blue. Pluto is still sort of black even after it stopped being a planet. This January astronomers have discovered Planet Nine, though they cannot see it yet. Centuries peering through their jolly telescopes and they’ve never seen this planet. Weird, really. What’s the colour of a planet you cannot see? Is this a Christmas cracker? Maybe, so far from the sun, it’s colourless. Ice-white? Ice-black?