Red diesel on vehicles on the roads?

Yes, you have the right to object. Then, they have the right to hold you there on suspicion, until their friendly on-call judge issues a search warrant. While they are waiting for the warrant, they will quite likely closely inspect your vehicle and write tickets for every violation they see.
Here in Montana, those license plates on my vehicles are MINE! When I get rid of a vehicle, I keep the plates. They might be allowed to take the registration stickers off the plates (I'm not sure about that), but the plates are mine, they can't have them.