Why is gold golden in colour?
D. Indumathi, The Institute of Mathematical Sciences, Chennai

Gold has fascinated human beings since time immemorial. The lure of gold has driven many nations, explorers and adventurers to go looking for this precious metal. Excellence in sports and athletics is all about winning "gold". But gold has also fascinated scientists--simply because of its colour. Most metals are shining and silvery---silver, iron, platinum, even reactive metals such as sodium, magnesium, aluminium, and exotic ones such as uranium, plutonium, polonium, etc. But gold is coloured, and coloured with such a lovely gleam of yellow. Why is it coloured?
You might have seen the elements lined up in rows and columns in the periodic table. With an atomic number of 79 (or with 79 protons in its nucleus and 79 electrons spinning around it), gold is in the last row of the periodic table that contains stable elements. Only four stable elements (mercury, thallium, lead, and bismuth) have greater atomic number.
You may also know that the rows in the periodic table tell us how many shells or orbitals of electrons are there around the nucleus. While hydrogen has just one, gold is in the sixth row of the periodic table and so has upto six shells of electrons orbiting the nucleus. In fact, just like hydrogen, gold has just one electron in the 6s shell (outermost shell). Then this should have been as reactive as hydrogen (or sodium, or any of the first-column elements of the periodic table).
You may have also learnt that the electrons in different shells or orbitals around the nucleus have different energies and so these are sometimes also called energy levels.
Because of the large number of protons in its nucleus, the electrons of the gold atom are subjected to an intense electrostatic attraction. In addition, the velocity of the electrons in the orbits are nearly half the speed of light, which is very high. Due to these effects, the outermost electron gets pulled towards the nucleus.
All this is true for a single gold atom, but also is true when many many gold atoms together form the solid metal. What happens in that case is that the overlapping of the energy levels of single atoms causes bands of energy levels to be formed. Hence, there is a 6s band, where the electrons from different atoms of gold in the metal can be found.  Because of the large numbers of electrons, some energy bands start to overlap. 
Now, what does this have to do with colour? We know that light is an electromagnetic wave that is visible to us. If the wavelength of light beomes larger, it becomes an infra-red or radio wave, and if its wavelength becomes smaller than visible light, it becomes ultraviolet light. Only the colours from violet to red can be seen by humans.
Now, when light falls on a surface, it gets reflected back into our eye.  Hence we are able to see the object. However, if the light falling on say gold has an energy equal to the difference between two energy levels in the metal, it is absorbed and the electron uses that energy to jump to a higher energy orbital. After some time, the light is emitted back, with the electron "falling back" into the lower energy state. Since given colours of light have fixed energies, this means that atoms in metals typically absorb and emit fixed colours of light.  In fact, for metals, most of the light does not penetrate the surface (it is opaque) and so most metals are highly reflective andd look white or silver. 
Sometimes, for some metals, the atoms absorb the light, but are not equally efficient in re-emitting it. This is called reduced reflectivity. While most metals have excellent reflectivity over all the visible spectrum, gold has reduced reflectivity at lower wavelengths, as can be seen from the figure (Note that the x-axis goes from higher to lower values of wavelength).
In contrast, the figure shows that silver loses reflectivity (that is, has increased absorption) at much lower values of wavelength, about 300 nm. This corresponds to wave lengths in the ultraviolet region, which is anyway not visible. Hence silver, like most other metals, shines silver.
What about gold? It absorbs most of the light about 500 nm or less, which corresponds to the blue region of the visible spectrum. Why does this make gold look yellow? To answer this question, we need to understand something more about colour and how we perceive it.
Modern color theory uses either the red-blue-greeen (RGB) additive color model or the cyan-magenta-yellow (CMY) subtractive color model. This just means that all colours can be generated by suitable combinations of these three primary colours. For instance, equal amounts of red and green make yellow, equal amounts of blue and green make cyan, and equal amounts of blue and red make magenta. Conversely, we can start with cyan, magenta and yellow, and get red-green-blue. See the picture on the back cover of this issue.
Complementary colors are pairs of colors which, when combined or mixed, cancel each other out (lose hue) by producing either white or black. When placed next to each other, they create the strongest contrast for those two colors.
Complementary colors may also be called "opposite colors." Which pairs of colors are considered complementary depends on the color theory one uses: If the primary colors are red, blue and green, when we combine them we get white light. The complementary pairs are magenta–green, yellow–blue, and cyan–red.
In other words, if blue is absorbed out of white light, its complementary colour, yellow, is left behind. How do we understand this? Looking at the colour wheel on the back cover, it is clear that if we remove blue from white light, red and green are left behind. You can see from the colour wheel that he combination of red and green makes yellow. In other words, when red and green light fall on our eye, it combines them and the light appears not double-coloured but a single colour, which is yellow. So the way we see colour is also strongly inflenced by the way our eye sees colour and how our brain interprets what the eye sees.
Now you can guess where this is going. We saw that blue light was strongly absorbed by gold. So when white light shines on this metal, the blue is absorbed, while more red and green are emitted/reflected. So the gold looks golden in colour. A simple accident of nature makes gold so different and so enticing, over which many wars have been fought.
It may be interesting to know the manner in which the velocity of the electrons plays a role: when the velocities are so high, ordinary mechanics is not sufficient. Einstein showed that relativistic corrections are important. These corrections are precisely such that they make gold shine golden, while silver remains silvery. Copper also has these effects (it also has one outermost electron, just like hydrogen, gold, and silver), but it has less protons and so the effect is smaller so that the colour that has smaller reflectance is not blue but blue-green. Hence it is not the pale yellow of gold but a darker orangered, which is the complementary colour to blue-green.
Complementary colours and art
In 1872, Claude Monet painted Impression, Sunrise, a tiny yellow-orange sun and some orange light reflected on the clouds and water in the centre of a hazy blue landscape. This painting, with its striking use of the complementary colors orange and blue, gave its name to the impressionist movement of art and artists. The classic painting is reproduced on the back cover of this issue of JM.
Vincent van Gogh was especially known for using this technique; he created his own oranges with mixtures of yellow, ochre and red, and placed them next to slashes of sienna red and bottle green, and below a sky of turbulent blue and violet. He also put an orange moon and stars in a cobalt blue sky. He wrote to his brother Theo of "searching for oppositions of blue with orange, of red with green, of yellow with purple, searching for broken colors and neutral colors to harmonize the brutality of extremes, trying to make the colors intense, and not a harmony of greys." The resulting masterpiece, Starry Night, is on the magazine's front cover.
--Compiled from several sources. Figures from http://www.webexhibits.org