A character's soul shade is a feature of their soul color that will be unique to only them and their soul partner(s). Your soul shade is something that will be discovered throughout the campaign, and like your soul class, is not chosen by the player. Your soul shade is a secret, and even when correctly guessed by a player, it is not to be shared with other players unless given explicit permission by the game master. While soul shades themselves do not grant extra abilities, they are free to be used for world building purposes once known in canon, and will give you the needed information to unlock a slightly different ability down the line.