I'm positively no expert, but a good friend who is big time, and I mean BIG time, into horse shows told me what you said is definitely not the case. Apparently the DOT cops stake out horse shows quite often since they catch a relatively high percentage of pickups running dyed diesel from their farms. If the giant Q-tip comes out clean, you're good to go. He told me that some over zealous DOT cops tried the fuel filter trick and got shot down big time in court.
There is nothing illegal whatsoever in running dyed diesel in a truck while it's operating on your property or farm. The tax is a "road tax". If it's not being used on a road, no tax need be paid. The court ruled that there was no possible way for the DOT cops to even remotely suggest that the truck was using dyed fuel on any road if the tank only contained non-dyed, taxed, fuel. These horse guys who travel quite a bit all know this he told me.
My question would be how do you know there is no more tinge of red in your tank? That's where I'd get nailed if I ever did that. Even if you run your truck until the gauge says it's empty and then run a couple more tanks of non-dyed fuel in it, how do you know it won't have some red tinge to it? That is where I think a real debate could occur.
