If you want to change one calendar's color, please see How to Change the Color of Your Outlook Calendar. If you have more than one calendar, you can change them to the same color together with the steps below:
Step 1: Click the "File" tab from the Ribbon;
Step 2: Click "Options" from the left navigation to open "Outlook Options" window;
Step 3: Click "Calendar" from the left navigation;
Step 4: Move down to "Display options" section, click the arrow to the right of "Default calendar color", pick a color you want, and then check the box of "Use this color on all calendars";
Step 5: Click "OK" to close the Outlook Options" window.