When you add your To Do List as an overlay on your calendar, you can apply colors to your To Do overlays to help them stand out from the rest of your calendar. In order to assign colors to your To Do overlays, you'll need to go to your calendar and click on the name of your To Do List from the Navigator. Follow the steps in the image below:
Any colors that you assign to your To Do List overlays will override the green "Start" and orange "Due" colors on your Tasks. When you choose colors for your To Do List Overlay, these colors will be applied to all Tasks from that List. Each To Do List Overlay can have its own colors.
Below is what it looks like when you use borders for additional color-coding on your To Do Overlays: