I have no excuse
Disclaimer: Harry and company do not belong to me.
Spell casting on the Hogwarts Express
"Fred gave me a spell to turn him yellow, but I don't think it'll work." The red-headed boy looked gloomily at the rat in front of him.
Turning to his companion, a bespectacled boy with messy black hair, he asked: "Want to see?" The black haired boy nodded eagerly.
At this point, the two were interrupted by a flustered-looking girl, who turned to Ron. "Have you seen a toad? A boy named Neville's lost one." The black-haired boy, Harry, and the redhead, Ron, both shook their heads.
It was at this point that the girl, whose name was Hermione, noticed the wand in Ron's hand. "Are you doing magic? Let's see then." she said slightly pompously.
Ron nodded glumly, although he didn't feel like trying to cast a spell that most likely wouldn't work in front of the girl. All heads turned expectantly to Scabbers The Rat, and the redhead took a deep breath, before intoning with a certain lack of style a brief couplet.
"Sunshine, daisies, butter mellow,
Turn this stupid fat rat yellow!"
To Ron, Harry and Hermione's surprise and amusement, the rat in question did indeed turn a bright, sunny shade of yellow. After a brief silence, the three teens burst out laughing, none of them aware of the consequences their action would have.
Several years later:
Sirius Black, an innocent man condemned to a life sentence in Azkaban because of his old friend's betrayal, stopped mid-rage as he stared at the recently discovered Peter Pettigrew, formally known as Scabbers The Rat. He was furious, but he had been rather put off by Pettigrew's appearance after his transformation.
"...Why the hell are you yellow, Peter?"