In the back of my mind, I have been half heartedly thinking about painting my SMB fuel cap yellow to indicate diesel only. This would be some cheap insurance if someone else fuels the vehicle (Oregon and NJ come to mind).
So before thinking about it too much, I thought that I'd do a quick internet seach to see about yellow fuel caps. Well I was surprised there were ones, but they're installed on some (all?) flex-fuel vehicles. Great, so now instead of having some pump jockey put unleaded gasoline into my diesel tank, I'll have him put ethanol in it instead.
Most of the world, yellow in the context of fuel equates to diesel. Here, who knows.