system
Given the following post, respond as if you're commenting on Reddit:
human
What about the atomic structure makes gold or copper their trademark colors but leaves most others a shade of gray?

Just what is it about those numbers of protons that makes such a drastic difference?
gpt
Well, gold, silver and copper make for a good comparison here, since they have the same electronic structure (same group in the periodic table). So you might expect them to have the same properties with each other at least, if not with the other metals. But they look quite different.

Most metals don't absorb strongly across the visual spectrum (but do reflect) so they have a shiny grey appearance. The energy difference between the lowest unoccupied energy level of its electrons, and the highest occupied ones, is in the ultraviolet range in silver. In gold, that difference is shifted down into the blue range of the spectrum, so gold absorbs blue light, reflects the rest, and appears yellow.

The reason for that shift in gold is actually Special Relativity. As things go faster, they acquire relativistic mass. The faster an electron 'orbits' the nucleus, the heavier it becomes (from the viewpoint of the nucleus or some outside 'stationary' observer like us). The more charge the nucleus has, the more attracted the electrons are to it, and the electrons are located more closely to it. In quantum mechanics, the more confined an electron is to a region of space, the higher its kinetic energy is. So heavier nuclei have faster-moving core electrons, and when you get down to heavy elements like gold, they move so fast that their speed gets to the order of the speed of light, and the effects of Special Relativity start to make themselves noticed. So they gain relativistic mass, and this in turn affects their motion as well. Which in turn also effects all the other electrons as well.

It's special relativity here that causes this shift in the levels; we know this because you can do the calculations on that energy gap without taking special relativity into account, and then you get the result that gold would indeed be silver-colored. So the yellow color of gold 'proves Einstein right' - which is pretty remarkable considering how far removed it is from the original phenomena Einstein was looking to explain. 

Copper is a bit of a different story, and can't be given a simple explanation as easily. The reason there is that it has a bit of a 'special' status by virtue of being in the first row of its 'block' in the periodic table (the d-block). The thing about that, is that the _outermost_ (valence) electrons in that case are closer to the nucleus, compared to the ones in the rows below it, for quantum-mechanical reasons (technically: the 3d orbitals lack a radial node). This, too, shifts the levels and causes it to absorb visual light. It also gives copper a 'second' oxidation state (Cu+ and Cu++) compared to Au and Ag, which only have Au+ and Ag+. 

The reason these colored metals occur in that group (and not others) is because of the specifics of their electronic structure. Most metals are in the d-block of the periodic table, meaning they have 1-10 electrons in the 'd' sub-shell. Scandium's group has 1 and Zinc's group has 10. But Copper's group _also_ has 10 d-electrons, and instead one less s-electron (which is the sub-shell that's usually lower in energy than d). It's energetically 'better' for it to have a filled d-subshell than a filled s-subshell.