using System.Drawing.Drawing2D;

    private void Form1_Paint(object sender, PaintEventArgs e)
    {
        var hb = new HatchBrush(HatchStyle.Percent50, this.TransparencyKey);

        e.Graphics.FillRectangle(hb,this.DisplayRectangle);
    }

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        // Apply opacity (0 to 255)
        panel1.BackColor = Color.FromArgb(25, panel1.BackColor);
    }

protected override void OnPaintBackground(PaintEventArgs e) {
  var hb = new HatchBrush(HatchStyle.Percent80, this.TransparencyKey);
  e.Graphics.FillRectangle(hb, this.DisplayRectangle);
}

protected override void OnPaintBackground(PaintEventArgs e) {
  var sb = new SolidBrush(Color.FromArgb(100, 100, 100, 100));
  e.Graphics.FillRectangle(sb, this.DisplayRectangle);
}