I made a post about this exact subject here

It's one of the rare hairstyles of the game, if not the rarest! It's at the bottom of the hairstyles chart so she can only produce the rainbow-haired child with the man with spiked blue hair. Sometimes there are also mutations so you could accidentally create the rainbow-haired girl with another couple but that would be really rare!

The light blue/dark blue hair is also rare It's near the end of the chart close to the rainbow-haired girl but there's more than one way to get a child with that hair.