system
Given the following post, respond as if you're commenting on Reddit:
human
What makes gold, gold in colour?

What causes the yellow colour of gold? While most metals in their reduced state are effectively colourless, gold (and copper) have colour beyond just being reflective. There are no things like organic chromophores in a metal, and the transitions that give metal salts their colours shouldn't be applicable in the neutral metal, as far as I understand. So, what makes gold (and copper, etc) special amongst metals?
gpt
In heavy elements like gold, the electrostatic attraction of the nucleus drags electrons so close that their velocity becomes a substantial fraction of the speed of light. (In an orbit, radius and speed are  proportional at constant momentum.) This means that relativistic effects become significant. This causes the 5*d* and 6*s* orbitals to contract. 

When they contract like this, the energy difference between the two orbitals also changes. The corresponding energy gap in a silver atom is equivalent to the energy carried by light in the ultraviolet spectrum. But in gold, it shrinks into the visible part. 

So electrons absorb visible light at a certain frequency as they transition from the lower energy state to the higher energy orbital. The visible spectrum minus this absorbed light is what we perceive as golf.