There is nothing "spiritual" about your aura. The body possesses an electromagnetic field and sometimes low levels of radiation is produced by your metabolism. This radiation can be picked up by instrumentation. The "colors" produced are not actual colors of light since the human body does not produced visible light, but "interpretations" of the various frequencies of energy coming off the body represented by the instrumentation. Since a change in a person's mood has an affect on their body, frequencies can be altered by your mental state.
Some people claim they can see it and see the various colors. Apparently--they said--if you hold your hand out against a white background and stare at the part between your thumb and index finger you can see your aura which goes out at about 1 centimeter from your hand. I personally have never been able to see it using this method.
What people are referring to when they say they can see your aura against a white background is sort of the same thing. If the difference in body temperature is much higher than the difference in the ambient temperature of where you are standing, the thermal radiation coming off the body becomes visible as it slightly distorts the air molecules around you for, as they say, a centimeter or so.