Off the top of my head I believe it's proverbs 4:18. There's no such thing as "new light" or "old light" in the bible - the only scripture they use to justify it is one that says the path of a righteous man (as contrasted with the path of a wicked man) is like a light that gets brighter as the day draws near.
Of course, any time there's a reference to a day drawing near, the JW's mind will immediately relate that to armageddon. In this context, however, it's clearly just talking about the increasing light from the sun as dawn approaches, nothing about armageddon is implied in any way.