Great post Avturboy.
If you are going to operate a misfuelling prevention policy then having clearly placarded fuel ports is a fundamental requirement.
Just to support this, and at the risk of really angering some unsuspecting pilots, technically, if each fuel filler opening is not placarded with the quantity and grade of fuel for that tank, the plane is not legally airworthy. A fueller could refuse to fuel it. In the real world this would seem extreme, but if I were the FBO, I'd cover my liability by requiring a copy of the flight manual limitations page, which would also show that placard.
Many checks and balances are already in place to prevent errors. You have to fail at a couple of layers to actually get an accident to occur (the Swiss cheese model). That's why every placard specified for the aircraft is reproduced in the flight manual.