C# - How to use variables within other functions. : I am trying to use variable rand from the Rnd function within the BtnRed_Click Function. The return does not do anything. And if i change the parameters for Rnd function i get an error.

Here my Code. 

namespace ColourPick_EDP2
{
    public partial class Form1 : Form
    {
        int RandomNum;
        int counter = 60;
        int rand; 

        public Form1()
        {
            InitializeComponent();
        }

        private void BtnStart_Click(object sender, EventArgs e)
        {
            BtnStart.Visible = false;
            Tmr.Tick += new EventHandler(tmr_Tick);
            Tmr.Start();
            TbxDisplay.Text = counter.ToString();
             

           Random rnd = new Random();
           RandomNum = rnd.Next(0, 7);
           Rnd(RandomNum);
            
            
        }
        
        private void tmr_Tick(object sender, EventArgs e)
        {

            counter--;
            if (counter == 0)
            {
                Tmr.Stop();
            }
            TbxDisplay.Text = counter.ToString();

        }
        
        private int Rnd(int rand)
        {
            int rand = rand;
            switch(rand)
            {
            
                case 1:
                    this.BackColor = System.Drawing.Color.Red;
                    break;
                case 2:
                    this.BackColor = System.Drawing.Color.Blue;
                    break;
                case 3:
                    this.BackColor = System.Drawing.Color.Green;
                    break;
                case 4:
                    this.BackColor = System.Drawing.Color.Yellow;
                    break;
                case 5:
                    this.BackColor = System.Drawing.Color.Purple;
                    break;
                case 6:
                    this.BackColor = System.Drawing.Color.Orange;
                    break;
            
            
            }
            return rand;
        }


        private void BtnRed_Click(object sender, EventArgs e)
        {
            if (rand == 1)
            {
                lblScore.Text += 10; 
            }
        }
        

    }
}
