Side Plate Luger

[anchor24]

If the side plate is a different color than the rest of the gun, and the number matches, what can be the reason the color doesn't match?
For example, is the metal different? Is the bluing method different on the small parts? Or can the side plate have been replaced with a same number side plate?
The store that is selling the gun is the Yankee Armorer, inventory number 4621. That will show the different color side plate:

gun. http://usgunexchange.com/dimages/or_7478.JPG

Thank you for all of your help look forward to hearing from you guys.