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Of course if there's no light around, there's no colour that you can see. On the other hand, the wall must have some property that makes it be blue. That property is still there in the dark.

Mike W.

Is Mike referring to the property of excited states, or the property that the states are excitable (via split d-orbitals or conjugated orbitals)?

I understand that anything that we see as coloured but not glowing is transmitting unabsorbed light — the complementary colour of the absorbed light — which is only possible when electrons jumping from ground to excited states.

What I'd like some clarity on is whether those excited electrons fall back to their ground state in the absence of light (another potential reason why colour does not exist in the absence of light). If the electrons do, then I guess the property Mike's referring to is the excitability of the electrons — which makes sense.

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The chemical property that creates colour is the ability to absorb light of a specific visible wavelength. There is more than one way to do this.

Mostly colour is caused by the existence of electronic transitions in substances that match the energy of some wavelength of light so when light hits the substance, some is absorbed by exciting electrons from a lower to a higher energy level. The higher energy levels will not be populated unless there is some energy input to excite the electrons. But the structure that enables this absorption to exist is present whether or not there is any light.

There are other ways a substance can create colour. For example, some compounds do it by interference: the physical structure contains features that have the same scale as wavelengths of some colour of light. When light hits the substance the reflected light creates interference patterns that can selectively eliminate some wavelengths. This is why oil films on water show rainbow colours and why the mineral opal shows beautiful iridescence. again, though, the cause of colour is present whether there is light or not.

In short, the cause of colour is sometimes a chemical thing (involving electrons being excited) and sometimes a physical thing (involving interference) but, either way, the property that creates it exists even when there is no light.

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