Asked by: David Pryce Morris, by email
The ‘colour’ of an object is the wavelengths of light that it reflects. This is determined by the arrangement of electrons in the atoms of that substance that will absorb and re-emit photons of particular energies according to complicated quantum laws. So tomatoes are red because the pigment atoms in the skin absorb photons of all energies except those that correspond to red wavelengths of light, which they reflect back to your eye.
But the colour that you perceive is about much more than the wavelengths of light. The visual cortex of your brain has evolved to perform lots of context-dependent corrections to correct for the colour and intensity of the ambient light. The effect is that the colour of an object has just as much to do with the colour you think it ‘should’ be.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.