Correct.  Federal law requires the dye concentration to be a minimum of 5 times the detection limit.  When a few gallons remain in a tanker or tanker and it is filled the concentration drops far below the detection limit, thus why the delivery tankers can haul both red an clear without a meticulous cleanout procedure.  Most tankers will attempt to haul a load of gasoline between a load of clear diesel following a load of red diesel, but it does not always happen.  I know our fuel guy will pump a few gallons of clear thru the line and dump it into the red tank on the truck before filling the clear barrel.  It costs them a little time and paperwork to file and get the road tax back on the flush gallons.

The guys saying it will linger for years have no idea what they are talking about.  I have switched a few tanks from red to clear by pumping until empty, add 20 gallons clear and slosh it around, pump it empty again, add 20 gallons of clear and have DOT check it.  When I say have DOT check it, I reached out to local DOT office and they sent a trained guy out to take a reading out of the bulk tank before we filled it with clear and every time they say it is good for road use.  The last one we did I kept a sample from the first 20gallons of clear and he said that was good, so after having done a couple now I would just drain the barrel and refill with clear and go on with my day.