World of Hearts is an original animanga fantasy roleplay set in the fictional world of Kardia. Kardia is a continent consisting of four different countries, each one bringing its own strengths--and weaknesses--to the world.
In the world of Kardia, each person is born with a heart--a physical manifestation of their mana, or their life force. Each heart comes in a different color: red, orange, yellow, green, blue, indigo, and violet. There are also those who are born with dual mana forces, often a mixing of their parent's heart colors. There are two other types of hearts: the null heart, which is a heart whose energy has been stolen from them, or a mechanical heart, which is used to replace broken, stolen, or otherwise dysfunctional hearts.
Each heart color gives their owner a variety of skills or magical abilities. They are often associated with certain personality types as well--although not all citizens of Kardia put that much stock into such things.