Prismwake Merrow

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2008-05-01
  Official Ruling
  You can choose any single color or any combination of more than one color. You can’t choose colorless.
• WWizards of the Coast2008-05-01
  Official Ruling
  An effect that changes a permanent’s colors overwrites all its old colors unless it specifically says “in addition to its other colors.” For example, after Cerulean Wisps resolves, the affected creature will just be blue. It doesn’t matter what colors it used to be (even if, for example, it used to be blue and black).
• WWizards of the Coast2008-05-01
  Official Ruling
  Changing a permanent’s color won’t change its text. If you turn Wilt-Leaf Liege blue, it will still affect green creatures and white creatures.
• WWizards of the Coast2008-05-01
  Official Ruling
  Colorless is not a color.
• WWizards of the Coast2008-05-01
  Official Ruling
  This ability won’t make an artifact stop being an artifact. It’ll just be a colorful artifact.

Rulings via Scryfall