|MadSci Network: Chemistry|
This is a hard question to answer as there are many layers to it. But assuming that what you are asking is "why is a green object green and not yellow?", then the simple answer is that "the energy levels of the molecules that compose it absorb photons at the complimentary wavelengths". Okay, that is not really much of an answer as it doesn't get to the heart of things. So, first of all, the way that all matter interacts is through the exchange of photons. That is, atoms and molecules both absorb and emit photons. This is the way that one atoms "knows" that there are other atoms out there, somewhere. (I should add that "bonds" are the internal method for the atoms of a molecule to interact.) Photons of light do not randomly strike an atom and are absorbed, though. The method of absorption requires an electron to change its energy state from one level to another. The size of the jump that the electron must take is very carefully defined by the energies of the first and second state. That is, to jump from the ground state to the next one for any atom requires a specific amount of energy. And since light is energy, only certain wavelengths of light will cause the electron to jump. Thus, you can imagine that if white light hits an atom, then the colours corresponding to those wavelengths are removed. The colour of the reflected light is changed slightly. With atoms, energy is absorbed at very defined wavelengths. With molecules, the situation is more complicated as there are many more energy levels and other interactions - such as vibrations or rotations - make the energy levels a little broader. The result is that instead of absorbing a single line, a whole bunch of wavelengths get absorbed - say, the entire blue end of the spectrum. In that case, the compound appears yellow as those are the reflected or transmitted wavelengths - the ones not absorbed. By the way, we perceive colour because of the structure of the molecules in our eyes which have energy levels tuned to respond to certain wavelengths of light. Our brain interprets the signals from these molecules and synthesizes the picture that is in our mind. Of course, this means that colour may be a very subjective and personal perception. But underlying it is the notion that electrons in molecules jump from one energy level to another in response to being struck by a photon of light. And only certain jumps are allowed. If you would like to read a book that explains the theory (called "quantum electrodynamics"), I would strongly suggest the very excellent book, "QED" by Richard Feynman. It is a little advanced but worth the time. Hope this helps.
Try the links in the MadSci Library for more information on Chemistry.