[tuzakey]

This is typically used for agricultural/off-road fuel which is not priced with road taxes and as a result much cheaper. Off road fuel is dyed red in the US. If you get caught running dyed diesel on road you will be fined. Thus the switch on the dash, when you leave the highway to drive on your farm you flip over to dyed fuel to save $$.

[Darkphibre]

Oh, fascinating! My first vehicle was the family's 3/4-ton Diesel '84 Chevy Pickup from the farm, and I'd forgotten it had an Aux fuel tank! This makes a lot of sense.

[Mauricebranagh]

You don't have farmers filtering the red out and selling it - its also whey a lot of UK farmers love Diesel Landy's