Gosh. Any bound particle has discrete energy levels. It can be as small as a single nucleus (as in the case of a single isolated atom), or it can be large as in

Gosh. Any bound particle has discrete energy levels. It can be as small as a single nucleus (as in the case of a single isolated atom), or it can be large as in an electron in a real-life material you see around. The mathematics is more complicated because the potential the electron is subject to is more complicated, but its essentially the same thing. An electron in a gold atom is identical to an electron in hydrogen atom and has no intrinsic property of color. Be it an electron in a populated conduction band, Cooper pair in a superconductor or an isolated electron, the wavelength of the radiation is still a collective behavior of the total system determined by the effective Hamiltonian particles see, not its constituent parts. An isolated atom is, albeit small, a composite system (with many energy levels corresponding to different internal degrees of freedom, meaning it can emit photons at many many different wavelengths which means electrons in atoms have an associated energy spectrum, not some particular color), and if you're looking at its physics are high energies sufficient enough to excite the nucleus too, a quite complicated one too. I should also add that as much as it is tempting to think that once you understand how a single isolated gold atom works you can understand the behavior of electrons in bulk gold, that's just not how it works, and is a subject of condensed matter physics.