The pointed responses are not really called for. However, I do understand one of the predominant trends on forums is to assume someone who asks about an illegal act might be someone who is looking for any chance to cheat or break the law (even though reality may be different.)
Today was just the first time I've actually heard of people doing this, because in the past, off-road fuel would likely not be a grade that would be usable in modern vehicles, such as Mexican LSD fuel, which would destroy a modern (past three years) diesel due to the added emissions.
Don't assume intent to cheat the tax man when all there was, was idle curiosity. Especially why people would use, what I assumed was a lesser grade of fuel, and risk engine damage. I pay my 20 cents/gallon just like everyone else does in this state.
I did check with a CDL friend of mine a few minutes ago. Some truck stops get people using the dyed stuff in vehicles, so if a clerk notices a pickup truck filling up with "refer" fuel, they will let the local DPS (Texas's equivalent to CA's CHP) know, and they will pull the vehicle over for a stick test.