That very thing happened to me once. I was crossing a major airport with the mighty Squirrel HT3.
The clearance read something like: "You´re cleared to cross the active runway behind the A319, Air So-and-So".
I advised that I wash´t familiar with that type and after some more hints the controlled cleared me to cross "behind that yellow aircraft". Not at all mocking me, that was indeed very helpful!
Tom