如何创建与已初始化的枚举一起使用的协议扩展？

protocol CustomStringConvertible {
    var description: String { get }
}

enum Color {
    case red, green, blue
}

extension Color: CustomStringConvertible {
    var description: String {
        switch self {
        case .red:
            return "Red"
        case .green:
            return "Green"
        case .blue:
            return "Blue"
        }
    }
}

let color = Color.red
print(color.description) // 输出: "Red"