The human body is a multi-level organism. It is generally accepted for discussion purposes that there are 7 layers in the auric fields. The inner most level is the physical body.
The layers are composed of energy at various frequencies.
Those who can see or sense auras can detect various conditions of the body by sensing the vibratory levels of the various levels and the disruptions, interruptions in same. The colors indicate frequencies. Increasing/decreasing brightness or lack of clarity may indicate areas being challenged, Dis-ease shows up in an outer level before it enters the innermost, the physical level.
In this training, we first learn to feel our own energy, then that of others; we practice detecting changes in energy; scan the body of a partner for signs of energetic change, getting and giving feedback as to what may be going on in the vicinity of the different energy level..
This ability to sense energetic changes can be used to scan many things in search of information: A directory can be scanned, after setting one's intent, to find a desirable entry, such as a website, a doctor, a realtor, a store, etc. It can also be used for scanning manuscripts for errors, detecting the veracity of statements, etc. This ability is used by dowsers to locate water, by archaeologists to locate buried ruins, and to find missing persons.
Those who notice that colors people wear may appear to compliment or clash with a person are unconsciously seeing the person's aura. Colors in the aura appear complimentary while absent colors may be disharmonious.
There are some photos of auric fields below: