Probably the subtle field you are seeing is not technically the aura, often it can lead to seeing it though, It is referred to by mediums as the subtle body, but may be just a trick of the light, a chi field or even heat. some cameras can detect these fields and interperet what you are seeing and maybe that is something to check into. look up kirilian photography, infra red cameras, and thermographics to better understand.
i think that it is most likly their aura. it doesnt mean that you have magickal powers or anything, its just you can see what kind of person they are and what mood they are in. so dont worry. i see the same thing and i freak my friends out because they think i have like magickal powers-which i dont-