Can we say when, where, and how the colours we see are formed? Are colours present like the flowers in a garden, or just an appearance? A simple, surprising fact, The Unified Principle of Colour, brings answers to age-old questions about colour and colour relationships. To the physics of light, and the biology of vision, a third discipline is added: the process of consciousness that enables perception. It shows that the observer, a living being, has an essential role in producing the colours we perceive.