Diesel is a blend of esters(oils) and those can vary in color, the final color of a blend isn't specified, pour point, cloud point, and cetane rating are. Pour point enhancers lower that(anti gel additives). Lighter grades of fuel, like #1, tend to be less colored. Premium diesel was often colored before the off road had to be dyed rule, and the hearsay was that you could never get the dye cleaned out. A pipeline services company spent almost of time and work purging their power engine tanks of dyed fuel because they were told they had to, but I think it was just to keep the question from happening and mucking up the operations by being shut down to determine if they were cheating using off road fuel for on highway engines.
If I remember it was cambridge England but it has been about 4 years now he did it for "farmers" so they can fill there trucks with 1 tank of fuel on the farm but what little you would save sure ant worth the ticket if caught and I'm sure the government was watching him if I saw him at a farm show I know they know to
That actually not true tractors require ulsd aswell and if you need it higher they will sell you a add in