"Nonsense, Theo." Silly boy. "You stick out like a raspberry among blackberries. If we are to mingle and meet with those from worlds other than our own, we must look the part of simple folk."

She pauses, as if to say something incredibly life-changing --

"When in Rome, do as the Romans do!"

...So maybe that isn't the most accurate way to use that phrase, but it's how she's going to use it.