De ce este aurul(pur)galben si de ce este cupru rosu?

Cum explică Einstein:
The colour of metals such as silver and gold is mainly due to absorption of light when a d electron jumps to an s orbital. For silver, the 4d→5s transition has an energy corresponding to ultraviolet light, so frequencies in the visible band are not absorbed. With all visible frequencies reflected equally, silver has no colour of its own; it's silvery. In gold, however, relativistic contraction of the s orbitals causes their energy levels to shift closer to those of the d orbitals (which are less affected by relativity). This, in turn, shifts the light absorption (primarily due to the 5d→6s transition) from the ultraviolet down into the lower energy and frequency blue visual range. A substance which absorbs blue light will reflect the rest of the spectrum: the reds and greens which, combined, result in the yellowish hue we call golden.

Warmly glowing gold,
What gives it that autumn hue?
Relativity.
Special relativity is also responsible for gold's resistance to tarnishing and other chemical reactions. Chemistry is mostly concerned with the electrons in the outermost orbitals. With a single 6s electron, you might expect gold to be highly reactive; after all, cæsium has the same 6s1 outer shell, and it is the most alkaline of natural elements: it explodes if dropped in water, and even reacts with ice. Gold's 6s orbital, however, is relativistically contracted toward the nucleus, and its electron has a high probability to be among the electrons of the filled inner shells. This, along with the stronger electrostatic attraction of the 79 protons in the nucleus, reduce the "atomic radius" of gold to 135 picometres compared to 260 picometres for cæsium with its 55 protons and electrons—the gold atom is almost 50% heavier, yet only a little over half the size of cæsium. Only the most reactive substances can tug gold's 6s1 electron out from where it's hiding among the others, and hence not only the colour of gold, but its immunity from tarnishing and corrosion are consequences of special relativity.

What keeps that golden
twinkle bright?
Mass increase near
speed of light!
Cuprul are culoare roșie-portocalie, aceasta fiind principala proprietate după care se deosebește de alte elemente.