I agree with Ryan-- these phenomena have a physiological
basis and if extreme can suggest an optical disease.
For example, I see a fogginess around objects in bright light--
but I also know that there's a slow cataract in one eye
causing this scattering of light. Nothing extraordinary about that.

However, I have been able to see a very light gray cloud around
people for decades. I mentioned this to a Qi Gong master
in a class with many experienced Qi Gong practitioners
who were training in a college of Chinese Medicine. Many
said they had the same experience. He said that this is not
Qi that, rather it is heat being seen by expanded perception.
So, Prana or Qi has a slight color to it that is distinctive and
visible to highly experienced meditators. Then, the question
becomes which Prana am I perceiving? It becomes very
complicated, very quickly-- lots of room for self deception.

Tigles that Ryan mentions typically appear in a dark retreat
when one practices Thogal. It is a practice one usually doesn't
enter until one has had solid experience in Threkcho, as
recommended and guided by a qualified Dzogchen Lama.
Typically, this would be one's last practice-- after Ngondro,
after Yidam, after Rushen, after Threkcho. That said, sometimes
the sequence of practice varries according to the Lama, the
student, and the tradition in question.