Drink the green potion.

You pick up the green vial, and turn it around, watching the green liquid flow slowly inside.

"I'm not sure about this", you say uncertainly.

"It'll be fine", George says. "Nothing has ever gone wrong with us."

"Except for that time that you went blind for an hour", Fred points out.

"You're right", George said. "That was pretty scary."

You put the vial back down on the table, afraid that some of the liquid got on your fingers.

"I think I'll pass this time", you say, standing up hurriedly.

You begin to walk out of the Great Hall, and Fred and George follow then, each one with a vial in a hand.

"Come one", Fred said pleadingly, holding out the green vial. "It's only meant to change your skin to a different colour. It can't harm you."

A look at their pleading faces makes you relent. You sit on the bottom step of the Grand Staircase, and take the green vial. You uncork it, and hold it away from you as a green vapour rises from the vial as a spiral. When it has dissipated, you drink it down. For a minute nothing happens. Fred and Georg stare at you avidly.

Suddenly, the twins yell in delight.

"What?" you ask, confused.

You look down, and catch sight of a green hand sticking out of your robe.

"Congratulations", you say, slapping each twin on his back. "Now, can you change me back?"

Immediately, Fred and George sober up.

"This has never happened before", George says. "We don't know how long it will last."

"What do you mean?" you yell. "I'm going to be stuck like this?"

"Don't worry", Fred says soothingly. "I'm sure Madame Pomfrey will be able to fix this in a second."