Follow the below steps to provide a unique color for the completed tab.
- Navigate to WooCommerce > Multistep Checkout > Advanced Settings.
- Enable the Show different color for completed tabs option from advanced settings. Disable it, if not needed.
- Click on the Completed Tab Background color textbox to choose a background color for your completed tab. Also, click on the Completed Tab Text color textbox and choose a text color for the completed tab.
- Once the changes are completed, click on Save changes.