Here is a picture of the labels I made for my fuel filler:
I printed those two labels on a little Brother label printer. It doesn't show up too well, but I painted the locking diesel cap Green as another subtle message.
You can also see a screw I added to the fuel door. I hang the fuel cap on that screw instead of letting it dangle on the paint:
You can also see the 'DIESEL FUEL ONLY' sticker that Ford put on the fuel door. You should be able to buy that from you local friendly Ford dealer.