Do you have color-changing lights on your tree, and one section is different from the rest? Or the lights do not sync together when you change light modes?

Fortunately, there's an easy fix. It has to do with the light cords being plugged together upside down.

Find the connection between the section that is not coordinating with the rest. Unplug and flip one of the cords the other way around. There should be a shape at the bottom of the cords that fits together when plugged in to one another. 

Here's a quick demonstration. As you can see, the two light strands are different colors when plugged in at first. One end of the cord is flipped (the right hand, in this case), and the sections now match when plugged back together.