zion-
this topic is very subjective, so, remember, this is only my opinion gathered from reading and experimentation...
an aura is the energy radiated from any type of object. the reason the aura is visible is that the object and the air surrounding the object are vibrating at different rates. i like to think of the aura as the 'gray area' between the object and the air. kind of like visualizing a river bed...you see a river (which is water, of course) meeting with soil...between the water and soil is mud...the mixture of both. it is the gradient between the two.
anyway, when people speak of seeing colors of auras, they are actually seeing the energy that a particular object is emmiting. one day, if i am feeling good and nothing is bothering me, my aura may be green. the next, i could have the flu and parts of my aura could be red or brown. i am sure that at one time or another each of us has been in a room with someone we just know is in a bad mood, but have not been told. the aura projects the mood just as does body language or speech.
it is possible to see the aura with the naked eye, just as you would start to get what i like to call, TV-vision, if you stare at a white wall for an extended period of time. it works the same way...when staring at a white wall, after a while each color pixel starts to become visible, the wall, you see, is not white, but made of many different colors of light. some people can 'tune-in' naturally, some people have to work for it...
hope this answers your question!