[saeho]

I know that metals such as gold and copper absorb the visible light area by the orbital contraction.
But when we looked at the metals in color, we found that they all had something in common that they had an s orbital with one electron in a stable state.
If there are two electrons in the s orbital, the resistance becomes more powerful, resulting in less orbital contraction, so if there is only one electron in the sobital, it becomes colored, right?

And each last orbital is copper: 4s1 gold: 6s1 francium:7s1.
Why isn't Rubidium colored with 5s1 as the last obital?
Shouldn't Rubidium, like these, absorb the visible light area by the orbital contraction?
Can you explain why?

I am still a high school student, so I don't learn the details, so let me know if there is anything wrong with it.
And because I am not good at English, I have used some translators, so please be aware of that.

[Mitch]

Gold is a bit unique since relativistic effects play a major part in its chemistry.

See: https://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#Color_of_gold_and_caesium

[chenbeier]

Additional copper also has a different color . All elements Cu, Ag and Au are in the same group of the periodic table.

https://www.flinnsci.com/api/library/Download/485982234af046b491724d3736c93c51

[Enthalpy]

I had in mind that it were a mere question of density of free electrons in the metal? Related with the cutoff frequency of plasmons.

Up to now I'm convinced that metals' colour is a molecular property. An argument in favour is that alloying changes the colour. Quite observable with with copper and gold. So atomic properties wouldn't give directly an answer.

Please take with mistrust, I don't have clear ideas about the topic.

[Flatbutterfly]

In order to understand the currently accepted reason why copper and gold are colored requires a familiarity of advanced quantum mechanics and “It is safe to say that nobody understands quantum mechanics.” (Richard Feynman).  And I see saeho that you are still in high school and quantum mechanics is usually only introduced around the second year of a BSc in chemistry or physics.  Band theory of metals may not even be taught in a BSc in chemistry.
Here is a part of the answer:
Of the metals only copper and gold are truly colored, but cesium has a yellowish hue, and many metals have a blue tint (e.g., lead).  Briefly the reason why Cu (Cu[Ar] 3d^10 4s^1)  is colored is that the sharp filled 3d band lies just below the Fermi level (in the partially occupied s band) and hence there are more electrons that can absorb high energy blue light and be promoted to the Fermi level.  The photons that have energies corresponding to red light are not so readily absorbed and consequently are reflected giving Cu is characteristic color.  Gold ([Xe] 5d^10 6s^1 is similar.  But why is silver not colored?  It is not because Ag (large 4d-5s band gap) that is out of place, but because Au is!  Relativity effects in Au contract the 6s band and raises the energy of the 5d band that leads to a small 5d-6s band energy separation as in Cu.
So, orbital contraction does play a part in the color of Au.

[Enthalpy]

Nice explanation, Flatbutterfly!

On this band structure for gold (OK, it's only computed) we see that some bands cross the Fermi level, making gold a metal.
https://arxiv.org/pdf/1203.4508.pdf
1) So due to the density of electrons and holes present, the absorption from 5d to 6sp would be much stronger than within the bands that cross the Fermi level, leading to the observable energy threshold in the light absorption?
2) And light absorption due to 5d to 6sp would also be significant as compared with the reflection due to electron conduction, which alone would make a colourless mirror?

3) Is the plasma cutoff frequency an explanation to be forgotten? [I have no affinity with it nor opinion whatsoever]

Thinking at the assumption that absorption from 5d to 6sp competes with reflection by mobile electrons, I search for changes in the electron mobility, here due to the temperature
https://hypertextbook.com/facts/2004/JennelleBaptiste.shtml
the resistivity drops /5 from room temperature to 77K and /100 to 4.2K, but I saw the same colour intensity for gold at 77K and assume that disappearance of colour at 4.2K would be known.
The absorption from 5d to 6sp, needing no additional momentum, must be independent on the temperature.
4) So would the conductivity hence reflectivity of gold at optical frequencies be limited by the electron's effective mass with little effect by their mean free path?

==========

While you're here, a different question:

Hall measurements combined with naive electron gas mental images result in about one mobile electron per atom in metals,
but
the heat capacity of metals is explained by the Fermi-Dirac distribution, which wants a very small proportion of mobile electrons, just around the Fermi level.

5) So what?

[Enthalpy]

Self-answering my 4)...
Seen figures like 30fs mean time between collisions for electrons in gold while 600nm light has 2fs period.
So at optical frequencies, the electrons' inertia limits the conductivity more than collisions do, and temperature doesn't change the reflectivity. Colour due to interband absorption can stay the same at cold.