Completed tasks in Outlook by default are marked with gray. If you want to use a different color, you can change to another color with the steps below:
Step 1: Click the "File" tab from the Ribbon;
Step 2: Click "Options" from the left navigation pane;
Step 3: Click "Tasks" from the "Outlook Options" window;
Step 4: Click the box after "Completed task color" and pick up a color for completed tasks;
Step 5: Click "OK" at the bottom to close the "Outlook Options" window.