Subject: what is the color of lightning?

I recently built a Jacob's Ladder and noticed that the spark is 
predominantly white in color with a small amount of blue.  I've heard that 
lightning/sparks emit signals covering most of the electromagnetic 
spectrum, but I'm not sure how it relates to color and if it is dependent 
on the source.  For my case, the source is a 60 Hz signal stepped up to 
about 12kV.  Is the color a consequence of the heat generated by the spark 
or the fact that the whole visible spectrum is being emitted?  Any 
information you could offer would be greatly appreciated.

Re: what is the color of lightning?