An engraving on glass has a frosted white appearance. On the white iPhone 4S it is subtle, just the textured mark cut into the glass. On the black iPhone 4S the appearance is sparkling white. Over time, since your phone is handled so much, the frosted color absorbs oils and fades, leaving just the smooth cut away area. A beautiful permanent etching. We have heard of customers using a sharpie or paint marker to apply a color fill.
If you would rather not part with your new iPhone you can purchase a replacement back from us. We will engrave it and ship to you and then you can easily install it onto your phone. Instructions are available on iFixit.com – Installing iPhone 4 Rear Panel
Back panels are currently available for the AT&T/GSM iPhone 4 and we are taking pre-orders for iPhone 4S.