Comment by extraduder_ire

Here, that's commonly called red diesel (despite them changing to green decades ago) and it's sold for agricultural use. There are a number of cross border smuggling operations where criminals remove the dye and resell it for somewhere between the two prices.

Though primarily done to trucks, there are occasional fuel tests done by police. Even if your tank is currently clean, they'll occasionally pull out the fuel filters and check those for dye.