Red Lightning Over Campbell Mesa, Coconino National Forest

Via Flickr:
The best I could gather is that light scattering is responsible for this rare phenomenon. It is the same thing that causes the sky to redden during sunrise and sunset, and makes it blue the rest of the day. Sunlight is temped at 5800 degrees kelvin, which equals white light. At most times of day, when the sun is overhead, blue light is being scattered the most. Sunlight has less atmosphere to penetrate during the day than when it is low to either horizon. This increased amount of atmosphere scatters out all the blue light and leaves us with those warm hues of red and orange.

As far as lightning is concerned, the same principles apply. The angle of the sun, the presence of rain or hail, and even dust in the atmosphere can cause that white light to be split and scattered, giving this effect. As you can see in the photo, there is a really pronounced precipitation shaft surrounding the lightning strike. Downdraft winds also kicked up a pretty good cloud of dust as this storm begun to move down into the deserts east of town. I'd never seen anything so amazing!