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Colour is your brain's way of telling you that the light you're seeing is of a particular wavelength (or combination of wavelengths).
To see an object in a certain colour means that it reflects (or emits) that wavelength (or wavelengths) of light. Which wavelengths it reflects depends on how the particular structure of its atoms and electrons interact with photons of light.
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I would further add that how we percieve a colour of an object depends on its background. Also negative afterimages, can make something appear to be a different colour by bleaching the colour receptors.
Here is a link to a quick afterimage experiment
afterimage
So in conclusion, objects emit certain wavelengths, how we interpret them determines what colour we see them. This interpretation can be changed depending on a variety of things. Colour is in the mind.