colour – the property possessed by an object of producing different sensations on the eye as a result of the way it reflects or emits light.

online dictionary definition

Crucial to physical colour is that requires an object to produce its effect, whether reflecting light or producing it. Observed or not, colour is the actual electromagnetic waves of energy that occurs when reflected or emitted. It is observed when it reaches the eye and interpreted by the brain of the observer as colour. The property of colour is not inherent in an object but is a characteristic of the material upon which light reflects. Always requiring a source material body, it is, loosely speaking, not an object in itself, but can be called a property of the interaction of an object and a particular form of energy called light.