Advent Calendar Day 5 - Polar Bear

“Ah yes”, murmured the Professor, “Ursus Maritimus! The beautiful Sea Bear! Now, I wonder…” (this to the small girl in his party) “… I wonder whether you can tell me what COLOUR is this bear’s fur?”
He looked absurdly pleased with himself. The small girl looked up at him before replying, “I expect that you think that I will say WHITE… But the Polar Bear’s fur is actually INVISIBLE-COLOURED!”.
The professor deflated, visibly. The girl’s mother (the professor’s daughter?) looked quietly pleased.