Everyone is born with a color they glow in. Some colors might look similar, but none is the same. Every color only exists once. And almost everyone has a soulmate, who- optimally- has a color opposite to theirs. Of course it's not always like this, but as long as the two colors aren't too close to each other everything works the same. Once two soulmates have found each other and showed their love to each other, their lips will turn into their color together permanently. For example, the lips of a red and a yellow person would turn orange if they were destined to be together, while they keep their own around them. However, those own colors can only be seen once the lips changed colors, meaning you never know if you've found your soulmate until you notice the change of color on your lips. Now, here's the real catch: you have to believe that this person truly is your soulmate, yet alone that you even have a soulmate- because some people don't, and never will. Yet all others can see the color change, even if you don't. So in other words, you won't notice anything if you've given up believing, despite everyone seeing that there's no reason to. And as long as both don't sincerely believe to be soulmates, they won't be able to see their own or their partners lip color, as well as their own colors. And that's why after a very particular night with someone, Light, who has given up hope a long time ago, is more than shocked to hear Matsuda excitingly scream at him during work:"Wow, Light! Look, your lips are all purple! I could swear they weren't like this when you left yesterday, who's the lucky one? Ooooh I bet you're red!"