Well Fuagf, Since all living things have active energy and generate different types of energy fields much of which will dissipate into the surrounding space. Humans generate many types of energy which has to go somewhere. Thermal, electricity, bio chemical, we vibrate and create wave energy. Thermal can be seen in certain frequencies by devices.
Light waves (which can both be perceived as waves and particles? <--gets strange there with the light slot tests) and the human eye has a narrow sensitivity within the broader frequency spectrum, but depends on the person how wide their sensitivity is.
I believe science has proven that living things emit energy in various ways and if a person has whatever energy flowing from them that falls just outside of 'most' regular people's sensitivities, who's to say that some people, and this is probably true can see outside normal ranges just enough to see this energy glowing a color to them.