Everybody has eyes that are primary colors (red, yellow and blue). When they meet their soulmate their eye colors "mix" and become a secondary color. For example, if two people are soulmates and one has red eyes and the other has blue, they both get purple eyes.
However, the mc is born with eyes that are already orange, green or purple, making it impossible for them to fall in love and stay in love forever. You can take this in any direction you want.
