How Do I Change Background Color Of Forms Every Second Using Threads?

You don't: threading is not a good idea here as you can't access Controls (and a Form is a Control) except from the thread they were created on: the UI thread. If you try, you will get a Cross Thread Operation Exception - or you will have to Invoke the UI thread to do it for you, which moves the execution onto the UI thread, making you use of threading superfluous anyway!

Use a Timer instead - the Tick event can change the form background - and forget about threading for the moment.

Try this code:

int red = 0, green = 0, blue = 0;
private void timer1_Tick(object sender, EventArgs e)
{
    Random rnd = new Random();
    red = rnd.Next(0,256);
    green = rnd.Next(0,256);
    blue = rnd.Next(0,256);
    this.BackColor = Color.FromArgb(red, green, blue);
}

<text>Try this Code

public Form1()
{
      this.BackColor = Color.Green;
      InitializeComponent();
      var timer = new Timer();
      timer.Interval = 1000;
      timer.Tick += new EventHandler(timer_Tick);
      timer.Start();
   }

  void timer_Tick(object sender, EventArgs e)
  {
      var colors = new[] { Color.Yellow, Color.Green};
      var index = DateTime.Now.Second % colors.Length;
      this.BackColor = colors[index];
  }