Colour is a puzzling phenomenon. Perhaps nothing could be more obvious than colour; but one thing that Modern Philosophy has taught us is that, concerning colour, things are not what they appear. We all naively think that things exist in the world with their objective colours. Grass in green; the sky is blue; coal is black. But in reality, colour is merely the way that things with certain objective properties—notably the ability to reflect, emit, or absorb electromagnetic radiation of particular frequencies—appear to sensory apparatuses of certain species-specific (and even individual-specific) kinds.