However, it is not necessary for a healer to see auras at all in order to do energy work. Everyone has his or her own way of perceiving subtle energy, and one way is not better than another. For example, many healers sense the energy field with their hands. I've heard of energy healers who do good work without sensing energy at all.
Nonetheless, it can be interesting and useful for a healer to see the biofield. Here's a site that gives detailed instructions for developing this ability: www.thiaoouba.com/seeau.htm.
Here's a page if you're interested in exploring aura color meaning. There are other ways to discern your own aura colors besides seeing them, and this page gives you one that's both fun and accurate.
You can learn two exercises for developing hand sensitivity to subtle energy here.It's quite exciting when you first learn to feel the energy. (And continues to be exciting, even after many years, from my experience.)
And you can learn about sensing chakras, another part of the energy system, here.
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