using System;

public class Program

{

    public static void Main()

    {

        //Actuall values from API

        //-11941643

        //-16776961

        int? GroupColorValue = -11941643;

​

        var hexvalue = GroupColorValue.Value.ToString("X8");

        var groupColor = "#" + hexvalue.Substring(2);

        Console.WriteLine(groupColor);

    }

}