By default, Outlook changes the color of overdue tasks to red and completed tasks to gray. If these colors aren’t to your liking, you can change them so that they standout better.
1. Click Tools and select Options.
2. In the Tasks section, click the Task Options button.
3. Click the dropdown for Overdue task color and select the desired color.
4. Click the dropdown for Completed task color and select the desired color.
5. Click OK to close the Task Options window.
6. Click OK to close the Options window.