That's probably my most common query too. I have been asked several times if I can remove the dye from red diesel! Many thanks guys, keep 'em coming.
The first time i tried to buy hardwood shavings from a supplier, i needed to make sure they were hard, and not soft wood.
The chap asked me what i was up to with them, and i said something like "chemical filtration", not thinking he wanted to hear about biodiesel and soaps.
Anyhow, he (thought he) knew exactly what i was up to, and started chatting. It seems that what we use to remove soaps will remove the dye (and i guess, possibly, the chemical marker?) from red?
I think the only way i managed to convince the chap was by explaining to him that my (legal) bio worked out a whole load cheaper than using (illegal) red!