Dyed diesel fuel for off-road use in the United States is red. The dye identifies fuel that is generally taxed differently or restricted to equipment such as farm machinery, construction machines, mining vehicles, and other off-road applications.
The U.S. Environmental Protection Agency requires red dye to mark off-road fuel. Solvent Red 26 is the standard dye, although Solvent Red 164, a chemically similar alternative, is also used. The strong color allows inspectors to identify the fuel even when it has been mixed with ordinary undyed diesel.
Red diesel is chemically similar to road diesel; the key distinction is its legal use and tax treatment, not a different engine principle. Using dyed fuel in an on-road vehicle to avoid road taxes is illegal and can result in substantial penalties. Color conventions vary internationally, so “red” is specifically the expected answer for U.S. off-road diesel, not a universal rule for every country.