Red 40.
Sylvia remembered entering an American specialty store in Dublin before her departure to Hogwarts. Whatever the name was, she didn't care as she wandered with her mother, and for the first time, without Selena before her disappearance. While waiting for her mother outside the bathroom, she found something interesting, and unusual, Cheetos Mac 'N Cheese, Flamin' Hot Flavor. Out of curiosity, she spend her money on it, and saw it was a vivid and cruel red. It looked rather radioactive, and was bland, yes, but that was the end of that thought.
Now, Sylvia wanted an elaborate first prank. And after some deliberation, thinking about potions, maybe a bucket of feces (though, that might be going too far), etc. she had the perfect plan. She first smuggled some money through owl to a wizarding clan of half-bloods she found back in Dublin, The Little Half-Blood Mischiefs, basically a gang from ages 8 to 11, herself the oldest and the founder.
"Shadow", well aka Johnny McCarty, a rich and plump red-headed eight year old boy, had brought 30 boxes of the flaming red instant pasta on her behalf, and it was send through multiple owls, since she also paid for that delivery fee. She got quite the stare as multiple owls crashed the Hufflepuff table, but ignored them, as the owls dropped the packages on her... ironically, she was eating pasta at the time.
She now had 30 packets of pure "Red 40", or Cheeto dust, and she smirked, as she took a large bucket of water, and set up her prank, smirking vividly, as the bucket contraption was finished. She then took all the packets of Cheeto dust she took beforehand from the instant mac n' cheese package and mixed it into the water before carefully lifting it up again.
Sylvia also had a camera to film this, and waited. After 10 minutes, the victim entered the corridor, and triggered the contraption. Sylvia roared with laughter as the victim was covered in a vivid red of Cheeto dust cheese juice, and had to fall back and keep laughing, wiping tears from her eyes.
