just *briefly* back on topic:
Although as stated, its just the same as regular stuff, but with some dye and markers.
I work in a scrapyard, a while back, the refuelling company cocked up and started putting red into our regular diesel store. whole thing had to be pumped out, flushed out and had a stupid amount of time spent cleaning it. Still had traces, enough that one of our lorries got dipped at the side of the road (some 2 hours from our yard) found a trace of red, and impounded it right there and then, left the driver at the road side, and took the lorry away.
After much fines debates and hastle we eventually got it back.
Not worth the bother imo. As much as i hate the ridiculous price of diesel. (but hey, doesn't everybody?)
Now back off topic:
Road tax doesn't exist!
Road tax in a fair sense (money paid by drivers and spent on roads) was abolished in 1937. Its a vehicle excise duty, based purely on emmisions. But the name road tax just stuck.