Your mileage may vary, but I only apply it to the white part. Otherwise the rest of my face would also darken and still not match. Just my made up logic and no real insight from any other source.
Remember that this is a chemical tan and not permanent. You still need a bit of sun to get your scalp to start producing melanin to help even things up naturally. I have been using the Jergen's once per day, but after a week, I see little results. It is darker, but it is up in the air whether it is the Jergens or the amount of sun I am getting. I am starting to up the amount to see if that helps increase it a bit. Also 10-20 minutes in the sun at a time with an SPF15 several times per day helps.
Hope this helps.