Well considering that for it to cause any certain amount of damage to a target and staying withing the electromagnetic spectrum as there main source of damage. It would be colourless as you wouldnt be able to see it, but you would see the air around the bolt ionise and crackle more than likley, which if you are going off our air supply being high on iron (less than 0.001% that is i think)and high on nitrogen and oxygen i would say a blue colour but very faint so in essence no coulour at all.
But personnally i think leds only look good on large minitures