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If this is true, I fail to understand why we see colour at all -- wouldn't any wavelengths absorbed by electron promotion be inconsequential to colour because there would an equal amount, and identical wavelength of light emitted when the electrons returned to their ground state?

The only way this would make sense to me is if the energy of the photon absorbed by the electron is given off as heat or something besides an identical photon when the electron returns to its ground state.

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  • $\begingroup$ See also physics.stackexchange.com/q/70478/162193 this very question was posed more than one time here or in Phys SE. The answer of Anna V is one of the reasons. But this leads already to absorption and emission to happen (and their spectra to be recorded. Consider that the most straightforward set ups include a detector along the light direction and perpendicular to it, in the two cases respectively) $\endgroup$
    – Alchimista
    Commented Mar 28, 2018 at 8:27
  • $\begingroup$ See also physics.stackexchange.com/q/70478/162193 $\endgroup$
    – Alchimista
    Commented Mar 28, 2018 at 8:29

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Not necessarily... it can return to the ground state in multiple steps, releasing different wavelengths each time.

Also, the excitation could be caused by something other than light in the first place (heat, for example).

Also, relaxation isn't instantaneous, so the emitted photon wouldn't really be "cancelling out" the color of the absorbed one as some time has passed.

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  • $\begingroup$ Add that emission generally happens at random direction, a fact that itself guarantees some absorption along the path. As such you cover all possibilities $\endgroup$
    – Alchimista
    Commented Mar 27, 2018 at 18:32
  • $\begingroup$ Also the direction of emission $\endgroup$
    – Eashaan Godbole
    Commented Mar 27, 2018 at 19:16
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For your understanding of how many ways an excited electron loose its energy see following Figure, which displays Jablonski diagram describing the electronic levels of common organic molecules and possible transitions between different singlet and triplet states (M. Sauer, J. Hofkens, and J. Enderlein. Handbook of Fluorescence Spectroscopy and Imaging).

![Figure 1: Jablonski diagram]!https://www.hindawi.com/journals/isrn/2013/230858/fig1/

If interested, please also read the article: !https://www.hindawi.com/journals/isrn/2013/230858/

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    $\begingroup$ Press help (button with ? sign) while editing and read relevant parts. $\endgroup$
    – Mithoron
    Commented Mar 27, 2018 at 19:03
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You are right about the heat, when electrons return to there ground state( directly or indirectly) a part or all of the energy can can be transformed to heat, this is called non-radiative relaxation.

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