[chris (OP)]

What gives lightning bolts their colour? Why should lightning sometimes appear to be green? Is it the same reason that the aurora is green?

Chris

[LeeE]

Hmm... blue or green?  Green could be from ionised oxygen.

[Ho-ho-ho]

I may be wrong but isn't lightning white? As for coloured lightning and again I may be wrong but I think the colour of lightning depends on atmospheric moisture/water vapour, pollution/aerosols, the time of day and the severity of falling rain or even hail.

It's (iirc) to do with scattering and absorption of different wavelengths. I'm not clever enough to try and explain this but depending on what's between the lightning and you affects the colour you see. One example of this is during a violent theunderstorm, you may have heard that the cloud appears green in colour. One theory for the green colour is that rain and or ice absorbs the red colours and the cloud itself absorbs the blue leaving the green. This then is what makes the lightning from that cloud system look green or blue.

An area of high concentrations of ozone can make lightning look blue. You also get yellow, purple, white and pink lightning. White lightning, as well as being the cider of choice for teenagers, is also lightning that occurs in a dry atmosphere

For examples of how the severity of rain and  the rain curtain can affect the colour of lightning, this link has some pictures:
newbielink:http://www.nightskyhunter.com/Lough%20Neagh%20Thunderstorm%20-%20April%2025th%2009.html [nonactive]

A quick look on the net gives this:

NOAA NSSL severe storms lab
"Lightning can appear to be many different colors depending on what the light travels through to get to your eyes. In snowstorms, where is somewhat rare, pink and green are often described as colors of lightning. Haze, dust, moisture, raindrops and any other particles in the atmosphere will affect the color by absorbing or diffracting a portion of the white light of lightning."
newbielink:http://www.nssl.noaa.gov/faq/faq_ltg.php [nonactive]

And this from the weather guys at USA Today:
"In researching this graphic, I came across several theories regarding the cause of colored lightning. Some posit that it is the concentration of gases at the point of the strike that result in different colors. This explanation doesn't ring true to me as the lower atmosphere is very well-mixed and would not likely show significant differences in concentrations to result in lightning coloration. David Cook from Argonne National Laboratory agrees, "There has to be a significant concentration of particular chemicals that create colors during ionization; those concentrations don't occur even in highly polluted air."
Another explanation that is sometimes offered is that it is variations in the temperature of lightning that result in the different colors. Again, David Cook weighs in:
Lightning light in the troposphere is white, and includes all solar visible wavelengths, no matter what the temperature of the lightning. The temperature of lightning actually varies little from the weakest to the most energetic of strokes and is equivalent to or higher than the temperature of the surface of the sun."
newbielink:http://blogs.usatoday.com/weather/2008/07/what-causes-col.html [nonactive]

[Bored chemist]

Lightning is blue partly because it's hot, and partly because nitrogen has an excited state that gives blue light when it decays.