How do I change the color of a series’ graph in a chart?

Click on the series’ graph and open the Presentation properties tab at the top of the window.  In the Appearance group, you can select a graph color from the graph style drop-down list, or enable more color options by selecting custom graph style.

If you want to re-use the selected graph’s color, you can save it as part of a style sheet, which you can easily apply to your charts.