Gold is yellow because ... of relativistic effects on microscopic level!

My chemistry in school was limited
I remember Chemistry class, when we had to calculate the interaction between two atoms. Atoms, it sounded so cool! I saw atoms everywhere (later turned out I saw eye floaters, which might not be very good). Anyhow, we did just the basic type, ¹Hydrogen, and I wanted to move on with ⁸Oxygen and ⁹⁴Plutonium and stuff. Well, at least ²Helium - the next after Hydrogen. C'mon teacher, I don't need goggles to do theory, it won't explode! But I never got a good answer. Until now.

Color is the absence of other colors
So what is color? Since sunlight contains all visible frequencies, it turns white. (It's actually off-white #fff5f2, according to this article). And when sunlight hits stuff, some photons get absorbed and some reflected, and the frequency of the reflected light is what we see as color. Nothing special.

Newton is benched
But that's just classical theory. Already with 2 electrons significant corrections have to be made to classical theory. In quantum theory, light isn't reflected by stuff, but a photon is absorbed and re-emitted by an electron. Upon absorption, the electron jumps some distance. How far a jump is determined by the interaction between other electrons and the electric (Coulomb) field of the nucleus. This means that with increasing atomic number the possible interactions get higher and it gets tough already with ²Helium. So far, researchers can model interactions of up to 4-5 electrons. Gold has 79.

The yellow color
Gold has six electron layers and the distance between the last two is hard to determine theoretically. Usually, the gap is calculated to be bigger than experiments show. The transition between these layers lies in the blue spectra, and so blue is absorbed and yellow is left.

Source: Russian magazine Science and Life