Lightning traveling through open air emits white light, but can appear in different colors depending on local atmospheric conditions. Distant lightning can appear red or orange the same way the setting sun does, due to moisture, haze, dust, etc in the lower levels of the atmosphere. Light emitted by lightning has a similar visible spectrum as sunlight (white light), so the atmosphere should shift the colors of both the same way - given there is enough distance between the lightning and the observer.
When lightning strikes an object or the ground, the lightning channel is often a deep red or orange color for its last ten feet or so above the ground or the target object. Lightning striking a tree will appear a bright, fiery orange/red color for the length of the channel traveling down the tree.
Lightning photographs: The hue of lightning channels in photographs is usually a function of the type of film, camera, exposure and/or recording media used. The same lightning channel can appear blue, purple, red or orange depending on the type of film, length of exposure, and other factors. Slide film is more likely to produce a more purple/blue image, while print film tends to give lightning a more yellow/orange tint.
BELOW: Lightning shot on some slide films (LEFT and CENTER) tends to appear blue or purple, while lightning photographed on most print film (RIGHT) tens to appear brown or yellow.
  
BELOW LEFT: The lower few feet of lightning striking an object will often appear orange or red. RIGHT: Lightning channels appear fiery red and orange when traveling down a tree.
 
Green Lightning?
Green/turquoise flashes and/or changing colors: A flash of light in the sky that lingers, pulses and/or changes colors is not lightning, but electrical arcing from shorted-out power lines. These arcs are called 'power flashes' and can be triggered by a variety of severe weather - including ice storms, high winds, tornadoes, or by a direct lightning strike. Electrical arcing, whether caused by lightning, ice or wind damage, is very intense, can be as bright as lightning, can illuminate the entire sky and can change color from blue, green, turquoise, red and orange. When lightning strikes an energized power line, an electrical flashover arc can result. Lightning-triggered flashover arcs usually begin during the strike and linger for a few seconds after the strike is over. See our article about flashover arcs for a more in-depth look.
Power flashes are often incorrectly referred to as 'exploding transformers'. Only a few power flashes are actually transformer explosions - most are caused by shorted-out lines due to broken, crisscrossed or fallen wires.
Below is an image of a power flash caused by falling tree limbs during an ice storm in St. Louis, Missouri:
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