The Violet Breath/Flame is essentially a cleansing healing flame, where it feels safe to use the element of fire.
I often don't draw it when I use it. I visualize a 3D vortex with an outer layer and inner layer, and what direction they move in changes, depending on what's going on. I feel at a loss for words right now, because I am realizing that I haven't really paid attention to when the directions change. I just know that sometimes the two parts are moving counter to one another, sometimes they move in the same direction, and sometimes it flows downward into the body and sometimes upward and out of the body, and sometimes it's down and out through the back. All I know it's like scrubbing a blackened pot. You put in lots of hot water and soap, and you scrub scrub scrub and then you take water out and put more in and sometimes more soap, and you scrub scrub scrub until the pot is restored. And sometimes you put the energy in, and you just let is soak before going back in and scrub scrub scrub with water going in and out.
I would also like to note that though we call it the Violet Flame there are flames in every color and each has their purpose. I often see it as an opalescent color of fire, white with other colors moving in out of it. It's rather a little like, fireworks, in that it depends on what you are burning that creates the color of the fireworks, like in the image sample shown below. *
When do you use it? When your intuition tells you too. When it feels right. When you have that gut feeling or a hunch that it's needed.
Why is it called the violet flame? Because violet is a soothing color that most people can handle. White flame is all the colors together as one. For some people that's too much. Their energetic bodies can't handle that. So they need something gentler, something that feels safe. "White light" can feel like the tunnel of death, or blinding, red can be seen as angry, etc etc. But violet flame. Just the sound of it rolls off the tongue in such a nice way, and violet is such a vibrant color without being too vibrant.
I don't always know why I am using particular colors or why different colors will dominate, as in be most present. For me, if I worry too much about why I am doing what I am doing I start getting too much into my head and I stop feeling. Doing this work is, for me, is about being fully embodied and feeling the energies flow, my energy, the boundaries and bridges, Reiki energy, and the energy of the person I'm working with. If I get too wrapped up in the mechanics I'm personally less effective.
Others, however, work differently. You do what works for you, and if you are interested in meanings of colors I recommend learning color theory, color therapy, and different cultural associations with colors. You can also simply stick with the ROY G BIV** connection with the chakras, especially while you are learning. Trust you will know what to do for your client and be willing to open a dialogue with your client, such as, "I'm seeing a lot of red, does that have any meaning to you?" Sometimes such conversations can surprise you and be quite edifying for the both of you.
**Red Orange Yellow Green Blue Indigo Violet
* An extra image of different metals and the colors they burn for the curious. :)