Severus Snape had never done anything like this before to anyone before, let alone to someone from his own house. But he was determined that Lucius Malfoy would go down this time - the git deserved it for all the crap he had put Severus through throughout the years.
So here he was, not exactly sneaking as the bathrooms were his own too, and sneaking would make him look suspicious, but sauntering nonchalantly down the narrow corridor toward the Senior Boys' Bathroom of the Slytherin dungeons. Glaring at a couple of third year girls who squeezed by him giggling, Severus continued on his way, feeling the glass bottle hit against his leg, buried deep in his left pocket.
Grasping the doorhandle to the room, Severus scanned the dim-lit corridor once before entering, thankful that no 6th or 7th year boy had decided that now would be a good time to take a bath. Closing the door with a simple locking charm, Severus glided over to the end of the room where a long line of stone sinks and gold framed mirrors lay against the wall, each sink occupied with the owner's wash-bag and temporary gold-plated name-carving; in a bid to be special and undoubtedly Slytherin. Only one word came to Severus' mind; tasteless.
In the very middle of the mirrors lay one particular large frame; labeled 'Lucius Malfoy; Head Boy'. Smirking, Severus picked up his black and silver wash bag stamped with the Malfoy crest, and zipped it open. Inside he found what he expected - a Mary Poppins style bag that had countless amounts of cosmetics inside. The ponce. Reaching inside and picking out the simple, slender white bottle of shampoo, Severus unscrewed the glass stopper and poured half it's contents down the sink.
Say farewell to your silver locks, Malfoy... Severus thought with a sly grin.
Putting the shampoo bottle onto the counter, Severus swiftly pulled out the bottle of potion from his pocket, undoing the stopper with his teeth. He poured it inside Malfoy's shampoo, watching it swirl and mingle until the entire consistency changed back to white. Smiling in triumph, Severus put the empty vial of potion into his pocket, placed Malfoy's shampoo and wash-bag back in their correct spaces and headed back to the door.
Muttering the unlocking charm, Severus walked casually out of the room and headed back down the narrow corridor, not looking back until he was outside of the castle.