35 ILCS 505/4d

All special fuel sold or used for non-highway purposes must contain only the dye Solvent Red 164 at a concentration spectrally equivalent to at least 3.9 pounds of the solid dye standard Solvent Red 26 per thousand barrels of special fuel except kerosene-type jet fuel sold for the propulsion of any aircraft. The dye must be added prior to removal from a terminal rack. The Department may also require all special fuel sold for non-highway use to have a marker added.