Change color on line graph when it reaches a certain index

[jbuoncri1]

Hello. I hope the title isn't confusing, but what I would like to do is to change the color of the graph from one color to another when it reaches an index.

For example, say my x-axis is from 0 to 100, and at I have two colors I'd like to use: blue and red. I would like the color from 0-50 to be blue and 51-100 to be red. How can I go about doing that?