""A lightning plasma is quite dense, because the degree of ionisation in the air near the lightning is high. It therefore gives off a lot of infrared, visible light and ultraviolet. A lightning flash consists mainly of light emitted at all wavelengths, plus line and band emissions at characteristic wavelengths from the atoms and molecules present in the surrounding air * mostly argon, oxygen, nitrogen and water. The lightning we see contains contributions from all these sources and the colour depends on the kind of atoms in the air.
"A discharge in dry air looks white because there are few strong visible lines those which exist are mainly blue and violet * and the bulk of the visible light is emitted in a continuous spectrum. If water vapour is present, then the hydrogen atoms in the water create a very strong red line (known as the Balmer alpha line) that can dominate the visible line emission. If this line is superimposed on the white background created by the other atoms in the plasma, it would explain the reddish appearance of some lightning flashes. The reddish colour will be easier to see at night. The variability in the amount of red in a series of lightning flashes is explained by variations in the amount of water in the air. In addition, water vapour or tiny water droplets are more easily ionised and their hydrogen atoms more easily excited than the hydrogen in large water droplets * the latter must first evaporate, which takes longer than the duration of a single flash. It is possible that large droplets were broken up during the long series of localised flashes reported by your correspondent, thereby creating an exceptional redness.""
Source and further information:
Read more: What causes purple lightning? | Answerbag http://www.answerbag.com/q_view/808240#ixzz1QabhpCPS
"The violet lightning color is due the presence of red lines of hydrogen spectrum from water vapor, it means, the violet lightning are quite common in the rainy storms at low altitudes. The nitrogen spectrum is preferably blue (consider the color of nitrogen laser) instead, so such color is typical for the lightning in dry conditions. Low frequency bolts contains the orange to red component coming from Rydberg atoms, which gives the green to orange color to such lightning. Such color is typical for ball lightning."