I have a MacBook 2012 13 inch, and decided to put stickers on the blank space on the left and right of the trackpad. I was then told that I shouldn't place stickers there, as there will be a "sunfade" left over on the laptop after a long time. I know that some of the adhesive might be left over depending on the sticker, but is is true that the shape of the sticker will be there like a tan line on skin if it is left there for too long?
You may want to read this: Do not use palm rest or keycap covers on a MacBook Pro with Retina display - Apple Support
And I do think there will be a different color left after time when you remove the stickers.
What difference does it make, it's your Mac.
With no stickers I see a circle mark (suntan mark like you describe) on the right side of my 2012 MBP 15"—It looks like the old spinning HD underneath created it. Now it is a SSD in there, but the mark remains.
Stickers on MacBook
