Auras are made of energy. They make fields of the energy energy being emitted which basically surroud the body. You do not necessarily have to always see them as they were solid objects and not merely fields of the energy which is being emitted and surrounding your body.
There is a stage which many of us have been into including myself, where we sense the energy of a person but are unable to define character vibration and even the emotional vibrations that emanates and translated in our brains' visual cortex (which is what makes the color).
This colorless aura or specifically colored aura is just you perceiving energy like a sonar translating energy to a image in your visual cortex.
This can be a matter of perception. In the modern days, many magic practitioners promote the exercise of energy perception through meditation and visualization just in order to be able to come to a self understanding of energy flow and perception, of course. In that way they can be witnesses and testimonies the way magic and energy flow work and what it has to do with the colors..
Also I would like to add that some magic practitioners may relate the third eye with visual .perception of energy