Fuel dyes are the dyes that are added to the fuels. In some nations specially European it is mandatory to dye a low-tax fuel. This helps in checking its use in applications that are intended for higher-taxed ones. While the untaxed fuels are known as "dyed", those that are taxed are typically referred as "clear".
To give an example, in U.K. the "red diesel", gas oil applied
for heating purposes is inexpensive than the heavier-taxed diesel fuel,
but it has a different fraction, and comes with more sulphur content.
Therefore using it in diesel engines is a damaging proposition both
environmentally and financially, as less money to the tax collectors.
That is the reason authorities want to deter it at any cost and addition
of Fuel dye is one of the methods to stop such malpractice.
Obviously, the dyes that are used must be soluble in hydrocarbon-based
nonpolar solvents i.e. effectively a solvent dye. A popular variety is
the red dye, a variety of diazo dyes. Types of red dyes include Solvent
Red 19, Solvent Red 24, etc. While Anthraquinone dyes are used to give
green and blue shades, it includes Solvent Green 33, Solvent Blue 35 and
lastly Solvent Blue 26.
It might be important here to note that due to technological
requirements, it is beneficial to mix a liquid with a liquid instead of
a powder dye.
Here is a list of countries where Fuel Dyes are needed.