Yeah, I like soulmate Au's very much if it ain't already obvious!
**"Everyone's soulmate isn't actually something set in stone and as such, there are many compatible partners out there for each person. ***
From the moment humans open their eyes, they can only see the world in black, white, and greys. Only when a compatible partner is found, does the person's world literally gain color. The more compatible the soulmate candidate is, the more colors will be visible. Consequently, a person can have multiple compatible soulmates to choose from though more than one partner is also possible.
However, they must love each other equally or their bond will weaken which will cause their partner to lose their color vision and to once again see the world in just black, white, and grey. Thus, each person can tell whether or not their partner's affection for them is strong or feeble.
Moreover, when someone loses their soulmate they will not only see a dull colored world, but they can actually lose their sight gradually or altogether (go blind) depending on how much they loved their partner. They can regain their sight and color vision if they find another compatible partner. Though in their initial recovery, the world will appear black, white, and grey but will gradually become colorful as the affection deepens between the pair.
I think this would make for a nice one shot or story. Could be a happy ending, a tragic one (damn, where are the tissues?!), etc, you choose!
But if you guys right a something with this, please tell me cuz I wanna read it!