He's too lazy to check past threads where the average price of rainbows always ranged from 5000-6000 gold.

Rainbow skill and dress blooms used to go for 50 flower gems but one day Nutaku decided to stop exchanging gems for items.

Before I see any replying posts paragraphs long that I wont read, do remember that Devil's Claw was on sale for 5000 gold, who is a 5 star but Nutaku (or Superhippo) staff thought she was a 6 star. Serial code status does not affect price.