Auras emanate from the energy of the chakras. Each of the seven major chakras has its own color and energy as well as its own influence; therefore there are at least seven possible auric layers to view.
Usually when someone states that they see the aura of another person, it is the color emanating from the crown chakra. That is the color most visible to the novice aura reader. For example, notice painting from many years ago such as the Catholic saints or ancient rulers -- most of them have a golden aura or plate painted around the head either on the crown or from the neck up to the crown which depicted the artist concept of what he was seeing around the subject. It is possible that over the centuries we have learned not to see the auras rather than that we are learning something new. Auras have been documented for well over 5,000 years in recorded history.
Anyone can learn to see the aura of anyone or anything, such as people, plants and animals. An excellent book that teaches of the energy of the auras that emanate from the chakras is The Celestine Prophecy by James Redfield. This book is written as a mystery but divulges many insights about energy and how it works. If you read it closely Redfield actually teaches the novice how to see the auric layers around all things.