Here in the Netherlands (and other countries in Europe) we have 2 types of diesel: normal and red diesel.

The red diesel is exacltly the same as the normal one, but with some 'dye'.
Why? Because the red diesel is very cheap (no tax on it) because it may only be used in farmer vehicles (john deere etc.).

Its off course illegal to use this red diesel in your normal car, and sometimes the customs check (at borders for instance) all diesel cars that come by.
They take a sample out of your tank, using the filling hole.

They say that you can see long after you used red diesel that you did.

First time offenders pay a small fine, couple of hundred euros, second or third time much more.

Know some people who use this red diesel, never been caught, and various vehicles run very well on them, no difference at all with normal fuel.

WVO is very uncommon over here, because its also illigal, and not cheaper (plus you have to make some modifications i believe?)

A couple of years ago there was discovered a big scam in Belgium. Some guy made some stuff to throw by red diesel, and the red color would go away.
He changed thousands of litres, and made a small fortune.

You can get red diesel at some pumps near farms, but they wont let you fill your car with it.
You can take it with you in jerry cans.