I'm just a cute little bunny, nothing to look at here.
This is actually the second time this has happened to Mario. Once when he was still living with Chloe (who was pretty much everything Mario isn't) I gave them a 2" x 1" butt of a carrot. It was huge and they had fun chomping down. I noticed later that Mario's entire chin (if bunnies have chins) was orange. It was only hilarious because Chloe, who also had a white mouth, had not a lick of orange color on her. She was quite the little lady and Mario, well, he obviously slobbers a bit when eating.
The orange mouth from the first incident was a little more dramatic (but it occurred pre-digital era and I'm too lazy to scan something now). Will try to get an image of it up later. The first time it happened I thought for sure Mario would be stained for weeks. But he apparently cares about his personal hygiene because both times he had cleaned himself off by the next morning.