Hello Kasteo,
There is no standardized dye that colours diesel fuel. Diesel sold to trucks on the highway in Canada is undyed. Diesel sold to farmers usually has purple dye added to the fuel (less taxes on this fuel).
This dye is almost always hydrocarbon based. Addition of a water based dye will usually not mix with diesel and will settle on the bottom of the holding tank. The same dye can be used in the plastic industry, as well as other areas.
Dyes are usually added at very low concentration (less than 0.01 percent), and typically has increased lubricity values (laboratory tested).
I hope this helps,
Eugene Dakin Ph.D., P.Chem.