Ravenclaw Harry is actually a Slytherin and Wormtail doesn't know his colours.
Harry glanced at the dead body lying on the floor next to him with some regret. This death could have been avoided. He made a mental note to avenge the fallen Cedric Diggory if he had the chance and it came at no personal risk. Harry passively let himself be lead to the headstone and bound to it, although he tensed his muscles as much as he could to allow for more slack later on. When the man's face came into the light, revealing himself as Peter Pettigrew, Harry grimly reminded himself to kill the man at the earliest opportunity. He watched with cold interest as Wormtail levitated a white powder, which Harry assumed to be bone, into the cauldron and then sliced off one of his hands. Harry took savage pleasure in his scream of pain. When Wormtail approached Harry, brandishing a knife and foolishly recited the next line of the chant, Harry silently gave him permission to take his blood, making a small movement to offer his forearm. He hoped to Merlin that Wormtail did not notice his ploy. When the rodent-faced man didn't notice Harry's cunning plan, he found it hard to believe anyone could be that stupid. Sending a silent prayer to Odin, Harry took his opening. "It's anticlockwise. You have to stir it anticlockwise."
Wormtail looked at him in confusion.
"That's the other way," Harry said helpfully.
"I know that!" Wormtail blustered; a hilarious sight in his squeaky outrage. "But why would you be helping me?" He asked suspiciously.
"Because I'm a Ravenclaw," Harry rolled his eyes sarcastically, "That's what we do. We go around correcting people that don't want to be corrected and generally just making stuck-up nuisances of ourselves. Just thought I'd continue the tradition."
"Oh, okay," Pettigrew said cheerfully and started moving the ladle in the opposite direction. Harry watched in anticipation as it made its second turn, third turn, forth turn…
Boom!
Harry smirked as the cauldron exploded in a ball of searing flames and Wormtail screeched with pain as he was set on fire.
"It doesn't even need to be stirred, idiot." Harry told the burning man scornfully. "And I'm a Slytherin. There's a reason I'm dressed in green." Harry sighed as Pettigrew fell to the floor, twitching pathetically.
"Well now I'm going to have to get out of these ropes myself." He told an amused Nagini in irritation. He looked mournfully at his wand; lying just a few metres away. "And it looks like I'll be doing it the muggle way."