I don't really care if a retcon took away the GL vulnerability to a color, being sniped by a repulsor blast is enough to take him out anyway, regardless of the color of the beam.
I haven't caught up much on DC lately, are the Lanterns invulnerable to attack if they can't raise a 'shield' in time now? Sure they can make matter and stuff with the rings, but can they see a beam of light coming and react to it before it cauterizes their neck-stump? I always assumed they were more-or-less mortal (or some kind of alien) except that the ring lets them make matter, energy, and junk.
I don't see any targeting computers in that lame face mask. Unless Martian Manhunter is there to relay tactical data via-telepathy (which doesn't seem to be in the stipulation of Green Lantern VS Iron Man), what defense does a Lantern have from a high powered light beam coming at it from extreme range? Because Iron Man can do that, he's got computers and stuff.
I agree that it more or less comes down to who gets the drop on who, but it seems to me like Iron Man has the edge here, unless Green Lantern uses another hero's tech (bylaws?) to 'cloak' himself from Iron Man's sensors so that Lantern can get the first shot.