Как убрать выделение кнопки

Бисячий синий прямоугольник мешает, как его убрать?

namespace piano
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button5_Click(object sender, EventArgs e)
        {
            SoundPlayer sp4 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\sol.wav");
            sp4.Play();
        }

        //________________________________________________________________________________________________________________

        private void button3_Click(object sender, EventArgs e)
        {
            SoundPlayer sp2 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\mi.wav");
            sp2.Play();
        }
    //________________________________________________________________________________________________________________
        private void button1_Click(object sender, EventArgs e)
        {
            SoundPlayer sp = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\noty-do.wav");
            sp.Play();
        }

        //________________________________________________________________________________________________________________
        private void button2_Click(object sender, EventArgs e)
        {
            SoundPlayer sp1 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\re.wav");
            sp1.Play();
        }
        //________________________________________________________________________________________________________________

        private void button6_Click(object sender, EventArgs e)
        {
            SoundPlayer sp3 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\fa.wav");
            sp3.Play();
        }

        //________________________________________________________________________________________________________________

        private void button4_Click(object sender, EventArgs e)
        {
            SoundPlayer sp5 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\lja.wav");
            sp5.Play();
        }

        //________________________________________________________________________________________________________________

        private void button7_Click(object sender, EventArgs e)
        {
            SoundPlayer sp6 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\si.wav");
            sp6.Play();
        }

        //________________________________________________________________________________________________________________

        protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
        {
            if (keyData == Keys.F1)
            {
                button1.ForeColor = Color.White;
                button1.BackColor = Color.Purple;
                SoundPlayer sp = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\noty-do.wav");
                sp.Play();
            }
            if (keyData == Keys.F2)
            {
                button2.ForeColor = Color.White;
                button2.BackColor = Color.Purple;
                SoundPlayer sp1 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\re.wav");
                sp1.Play();
            }
            if (keyData == Keys.F3)
            {
                SoundPlayer sp2 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\mi.wav");
                sp2.Play();
            }
            if (keyData == Keys.F4)
            {
                SoundPlayer sp3 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\fa.wav");
                sp3.Play();
            }
            if (keyData == Keys.F5)
            {
                SoundPlayer sp4 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\sol.wav");
                sp4.Play();
            }
            if (keyData == Keys.F6)
            {
                SoundPlayer sp5 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\lja.wav");
                sp5.Play();
            }
            if (keyData == Keys.F7)
            {
                SoundPlayer sp6 = new SoundPlayer(@"C:\Users\MD\Desktop\piano\piano\piano\Resources\si.wav");
                sp6.Play();
            }
            return base.ProcessCmdKey(ref msg, keyData);
        }

        private void button1_EnabledChanged(object sender, EventArgs e)
        {
            
        }
    }
}