Rare colors for mermaid tails:
Silver: I've only seen one all silver tail. Even as an accent it's pretty rare.
Brown and bronze: I'd love to see brown as the main color instead of an accent.
Yellow: most go for gold instead of yellow, but yellow really pops in the water. Think of the yellow tang!
Muted yellow-green: common on real life fish, not so much in mermaids.