If you had a bit for every time you've hit an evil-minded pony with a magical item to make them repent and see the light of friendship, you'd have at least two or three bits. Which isn't a lot, but it's a place to start.
You don't exactly have your Element of Loyalty handy, but you do have a bucket of water that may or may not have traces of some kind of curse in it.
It's a far cry from the kind of purifying energy you'd like, but maybe when curse meets evil, like and like will cancel each other out. It's worth a shot.
"Enough stalling, le—"
You whip the Bucket out from your Inventory and fling the contents over her, catching her full in the face with her mouth open mid-word.
She hacks and sputters as she stumbles at you half-blind. You sidestep and let her crash into the wall. "You imbecile," she says between coughs. "Do you have any idea what you've done?"
Actually no, but you have a hope. "Cancelled out your evil with cursed water?"
The other Rainbow Dash laughs, a dry thing despite her recent drink. "No." She bangs on the wall, opening a secret compartment. Inside are a dozen slim vials of vibrant fluid in the seven colors of the rainbow. She grabs one, pops off the cork, and drinks it. You swear you can actively see the orange part of her mane grow slightly more vibrant. "No, you forced me to waste one of my better samples."
She blinks the water out of her eyes and stands up straight.
"A clever trick." She flicks a piece of pond weed off her shoulder. "But it's going to take more than a piddly little curse to stop me."
"It didn't work?"
"No, you ignorant fool. You can't neutralize a strong acid with a weaker one. The same applies to magic. The spirit in this water still has some Chroma, but it's so weak compared to mine it's like the buzzing of flies to me." She chuckles and smiles. "In fact, I—what's this?"
The tip of the hoof she was about to gesture with has gone white. Not a clean true white either, but a dirty washed-out pale grey. As you watch the greyness starts to spread up her hoof.
"What? No! Stop! You can't do this! You can't!" she screams at her own rapidly greying foreleg. "I'm stronger than you!"
It moves to her barrel, growing faster by the moment like some kind of terrible fungus.
"No! I won't let you ruin my work! I'll burn you out myself if I have to!" She reaches for the rest of the vials and starts tearing out their corks and downing them as quick as she can. With each swallow her colors flare for a moment before the relentless grey reasserts itself.
She runs out of vials before the curse runs out of strength.
Your doppelganger wails in anguish as she pounds at her reflection in the polished side of a machine. Her fur has gone completely grey, a shade that reminds you of corpses and fog. Even her once vibrant mane only runs the spectrum from white to black.
"It can't be, it can't! How? She was barely alive, she should have been crushed by the might of my... of my..." She sinks to the floor as monochrome tears bead on the edges of her eyes. "All my hard work... My ascension! I was so close!" Her sob transforms into a snarl as she turns to you, blind fury in her eyes. "You! This is all your fault! I'll kill you for this!"
You feel as though you took a calculated risk, but Faust are you bad at math.