To customize how a chart looks, first go the to Setup view mode. From here, you can select which lines are shown in your chart. Use Add data to add more lines to your chart. The list here can be as long as you want, but you can only select 20 lines at a time. This allows you to easily switch out lines without having to look them up each time.
To change the color or Y-Axis settings of a line, select Customize from the action menu (click the three dots). You can also directly click on the circular color icon to quickly open the customize screen.
Using the customize screen, you can edit the color and Y axis settings of your chart line. The color and Y-Axis settings are only used for the chart you are currently editing, but they will be applied to all locations where that chart is used. This way, you only have to make each change once.
Tip: Try using the same chart on as many locations as possible to reduce the amount of chart management you have to do.