Here is the scientific answer.
The color created by any electro-magentic discharge is directly related to the frequeny of the EM (electro-magnetic) discharge. The equation is E = c / lambda or E = hc / lambda where h = plank's constant and lambda = the wavelength of the light and c = the speed of light.
Give that the atmosphere is filled with multiple elements and compounds the color of lightening is directly dependent on the amount of energy discharged and the composition of the air.
This is because the excitation state of any give molicule or its sub components is direcly corelated to the incedent or input energy. Given that elements have orbitals and molicules generate orbital hybridization that deturmine the possible output energy based on the input energy. Which based on the first equation deturmines the visible "color" or wave length lambda (lambda = c/E) . The wavelength of light (or Electro-magnetic discharge) direcly effects the "color". As the elements in the atmosphere preferentially generate (red, green and blue light the sum of which appears white when combined) (like a tv set); lightening which is a high voltage EM pulse generates white light by stimulating all elecron states of the molicules and atoms in the air.
Lightning takes on a range of colors, depending on conditions in the clouds and in the air.
Blue lightning within a cloud indicates the presence of hail.
Red lightning within a cloud indicates the presence of rain.
Yellow or orange lightning occurs when there is a large concentration of dust in the air.
White lightning is a sign of low humidity (amount of moisture in the air). White is the color of lightning that most often ignites forest fires.
Sources: Burroughs, William J., et. al. The Nature Company Guides: Weather, p. 240; Field, Frank. Doctor Frank Field's Weather Book, p. 201.