I actually know a bit about auras, due to a lot of reading on healing. It turns out seeing auras is actually very helpful to the healer. This is what I believe based on what I've read, and what my eyes have witnessed.
Auras exist in any living body, because the aura is the radiance of the life force of that body. The Aura colors change in humans and animals, because we change, thus our life force changes.
Example: A man who follows a path of evil, greed and lust has a burnt brown and black aura clouded with a grey smokey like aura. This is because his spirit or soul radiates an evil life force. he brings evil where he goes if his will is strong enough.
A man who follows an absolute just path would have a royal purple aura streaked with a golden light, because his spirit radiates pure justice.
A man who heals people out of love, much like myself, has a green aura because his spirit radiates a light of pure love for all things.
The color of the aura is determined by the heart and mind of the being and the light its soul emanates.
That of plants is different, Since plants do not think, nor act upon mind. their aura is simply their spirits, and the brightness is their life force.
To see auras, you must practice.
Here is a good exercise.
draw an eye on your targets forehead (if its a person) if not its fine. draw it on the spot where the third eye is, this is where the light radiates the strongest, this and the heart chakra. blur your vision until you can see two drawn eyes and focus on the spot between them.
after a fair amount of practice, you wont need to draw the eyes anymore. For best results, practice with your target against something white, it makes the colors more apparent.