Winning Colorado would be one of the (very) few realistic scenarios where Trump could overcome the Democrat electoral vote advantage without winning Pennsylvania. It also assumes Trump victories in FL, NC and OH, all of which have become more likely over the course of the past three weeks.
Trump's biggest threat right now is not Hillary Clinton, whose mountains of money and permanent campaign infrastructure will certainly help her. His biggest adversaries are his own character flaws and his own big mouth. Tonight's debate could make a great deal of difference, depending on how he comports himself.