Version 1:
You can only see real color when you meet your soulmate. Like, you are a dark color, from your clothes to your hair, until you meet your soulmate. But you don't know it until you kiss them.

Version 2:
The first thing your soulmate ever said to you is tattooed on your wrist.

Version 3:
A red string appears on you and your soulmate's finger when you both turn 16.

Version 4:
Whatever you draw on your arm appears on your soulmate's arm as well.