Fuel dyes are dyes added to fuels, as in some countries it is required by law to dye a low-tax fuel to deter its use in applications intended for higher-taxed ones. Untaxed fuels are referred to as "dyed", while taxed ones are called "clear" or "white".
The dyes used have to be soluble in the fuels they are added to and therefore in hydrocarbon-based nonpolar solvents ("solvent dyes"). Red dyes are often various diazo dyes, e.g. Solvent Red 19, Solvent Red 24, and Solvent Red 26. Anthraquinone dyes are used for green and blue shades, e.g. Solvent Green 33, Solvent Blue 35 and Solvent Blue 26.
It is advantageous to mix a liquid with a liquid instead of handling powdered dyes into a liquid.
The pure dyes found in modern liquid petroleum dyes are essentially longer alkyl side chain forms of traditional dyes and normally multiple chain length variations of the chromophore are found within a typical commercial liquid petroleum dye. For instance, Sudan Red 462 is essentially Solvent Red 19, with the ethyl side chain replaced by either a 2-ethylhexyl or a tridecyl side chain. The longer branched side chains improve solubility dramatically, but in some cases the high solubility prevents the dye being isolated as a crystal, except at very low temperatures. The high solubility liquid dyes originated with Morton International and BASF (ACNA Italy) as the primary inventors. For instance, Morton International created Solvent Blue 98 as a high solubility form of Solvent Blue 35. BASF created Solvent Blue 79 as its high solubility form of Solvent Blue 35. In some cases it is possible, with normal solvents - e.g. xylene - to prepare stable (to -20C) solutions at 65% "solids" content. The original powder dye form of the chromophore would not be soluble beyond 2% in xylene.
Only a few refineries worldwide still use powder dyes for colouring fuels, as ultimately they are still lower cost per active molecule of dye chromphore than the modified forms. They have significant handling issues and health and safety issues that inherently arise from the handling of azo dyes (reds/yellows/green mixes).
Aviation petrol is dyed, both for tax reasons (avgas is typically taxed to support aviation infrastructure) as well as safety (due to the consequences of fueling an aircraft with the wrong kind of fuel).
Read more about Fuel Dyes: Fuel Dye in The European Union, Fuel Dye in North America, Fuel Laundering, Dyes Used
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