First things first.
What you interpret as colour is a result of signals coming from receptors in your eyes. Each type of receptor (red, green, blue) is excited by a different amount from different frequencies, thus allowing your brain to differentiate between frequencies by combining signals from the different receptors to perceive colour.
The proper radiometric (i.e. eye and brain-independent) way to pose questions pertaining to colour is through the concept of a spectrum. A spectrum defines the energy as a function of frequency of a given optical (or electromagnetic signal).
Thus when we say an object is brown - what we really mean is that the frequency content of the light waves is such that we perceive it to be brown based on the signals from the colour receptors in our eyes. If we were to measure the spectrum directly (say, using a spectrometer), we would instead get a curve that showed energy versus frequency.
Now the question becomes, why do objects reflect a certain spectrum? This depends on the properties of the object.
Emitted light is easy to study, because it depends ONLY on the object - this is why the sun has the colour that it does, and why metals in flame tests emit the colours they do. Reflected light however depends on both the object AND the manner in which it is illuminated. For objects that do not emit their own light, the spectrum we see is due to a combination of the
illumination spectrum and
reflection spectrum.
So what determines the reflection spectrum of an object? For bulk solids like wood,
it is due to the characteristics of the chemical bonds in the material, and not just the elemental constituents themselves. Consider the example of diamond and graphite - both are comprised of pure carbon. They exhibit different optical properties because the carbon-carbon bonds have different characteristics in each solid.
In short, the colour of something reflects the atomic and molecular nature of the bonds that hold the object together - this is why spectroscopy is such a powerful tool when it comes to analysing materials (see for example
http://en.wikipedia.org/wiki/Raman_spectroscopy).
Claude.
