When using the OrderPrinter from Shopify for the store, then the page which is not part of the actual invoice is grey.
To be more specific with an example:
Go to your POS iPad App. Add one Item. Click all the way through to the Payment-Complete Screen. Open the OrderPrinter App. Let's use the standard OrderPrinter Template, or any other template. It has this grey area on the bottom if the page is not 100% filled up.