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I am confused with the color of cesium. Some websites and books say that it is a silvery white metal, and some say it has a golden colour, and some say it has a silvery golden colour.

Among these colours, which is the true colour of the cesium? If it has a golden or silvery golden colour, why does its colour deviate from colours of the remaining alkali metals (i.e., silvery white colour)?

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A picture, specifically from WP user Dnn87, beats a thousand words. The gray background is ideal for bringing out the true golden color of caesium metal.

enter image description here

The color comes from plasma oscillations, in which electrons in a conducting medium spontaneously oscillate due to inherent instabilities. These may be traced to the electromagnetic interactions as described by Maxwell's Equations. In most metals the plasma oscillations are at too high a frequency to interact with visible light, but in caesium the plasmonic frequency is lower (due in part to the large atomic size and only one conductive electron per atom, which lowers the electron density); thus the absorption range enters the blue/violet region of visible light.

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    $\begingroup$ I do like this video showing caesium metal being distilled - it is both extraordinarily beautiful and extraordinarily dangerous! Interestingly, some portions appear golden, and others appear silvery. It's possible that sufficiently thin layers of caesium (maybe a few dozen atoms thick) can reflect enough light to look metallic, but not absorb enough blue light to appear golden. $\endgroup$
    – Nicolau Saker Neto
    Commented Sep 15, 2021 at 14:34
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    $\begingroup$ @nicolau the article you reference de-emphasizes the relativistic effect on electron orbitals for caesium, in favor of the plasmonic effect. $\endgroup$
    – Oscar Lanzi
    Commented Sep 15, 2021 at 14:50
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    $\begingroup$ Hmm, caesium vapor absorption spectrum is not so much decisive to support the plasmonic effect. $\endgroup$
    – Poutnik
    Commented Sep 15, 2021 at 18:33
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    $\begingroup$ You don't have plasmons in the gas phase. They are a property of the condensed metal. $\endgroup$
    – Oscar Lanzi
    Commented Sep 15, 2021 at 18:38
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    $\begingroup$ I know, that is why I have posted that. :-) $\endgroup$
    – Poutnik
    Commented Sep 15, 2021 at 19:36

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