How to change the border color of a GroupBox

Create the event below ...

private void groupBox1_Paint(object sender, PaintEventArgs e)
    {
        GroupBox box = sender as GroupBox;
        DrawGroupBox(box, e.Graphics, Color.Red, Color.Blue, Collor.Yellow);
    }

And the method responsible for changing the color of the border and text below ...

private void DrawGroupBox(GroupBox box, Graphics g, Color textColor, Color borderColor, Color backgroundColor)
    {
        if (box != null)
        {
            Brush textBrush = new SolidBrush(textColor);
            Brush borderBrush = new SolidBrush(borderColor);
            Pen borderPen = new Pen(borderBrush);
            SizeF strSize = g.MeasureString(box.Text, box.Font);
            Rectangle rect = new Rectangle(box.ClientRectangle.X,
                                           box.ClientRectangle.Y + (int)(strSize.Height / 2),
                                           box.ClientRectangle.Width - 1,
                                           box.ClientRectangle.Height - (int)(strSize.Height / 2) - 1);

            // Coloque a cor do background aqui
            g.Clear(backgroundColor);

            // Draw text
            g.DrawString(box.Text, box.Font, textBrush, box.Padding.Left, 0);

            // Drawing Border
            //Left
            g.DrawLine(borderPen, rect.Location, new Point(rect.X, rect.Y + rect.Height));
            //Right
            g.DrawLine(borderPen, new Point(rect.X + rect.Width, rect.Y), new Point(rect.X + rect.Width, rect.Y + rect.Height));
            //Bottom
            g.DrawLine(borderPen, new Point(rect.X, rect.Y + rect.Height), new Point(rect.X + rect.Width, rect.Y + rect.Height));
            //Top1
            g.DrawLine(borderPen, new Point(rect.X, rect.Y), new Point(rect.X + box.Padding.Left, rect.Y));
            //Top2
            g.DrawLine(borderPen, new Point(rect.X + box.Padding.Left + (int)(strSize.Width), rect.Y), new Point(rect.X + rect.Width, rect.Y));
        }
    }

Changes the Paint event of your GroupBox to call groupBox1_Paint.

You need to create a custom control.

Create a Class in your project and use the code below:

public class MyGroupBox : GroupBox
  {
    private Color _borderColor = Color.Black;

    public Color BorderColor
    {
      get { return this._borderColor; }
      set { this._borderColor = value; }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
      //get the text size in groupbox
      Size tSize = TextRenderer.MeasureText(this.Text, this.Font);

      Rectangle borderRect = e.ClipRectangle;
      borderRect.Y = (borderRect.Y + (tSize.Height / 2));
      borderRect.Height = (borderRect.Height - (tSize.Height / 2));
      ControlPaint.DrawBorder(e.Graphics, borderRect, this._borderColor, ButtonBorderStyle.Solid);

      Rectangle textRect = e.ClipRectangle;
      textRect.X = (textRect.X + 6);
      textRect.Width = tSize.Width;
      textRect.Height = tSize.Height;
      e.Graphics.FillRectangle(new SolidBrush(this.BackColor), textRect);
      e.Graphics.DrawString(this.Text, this.Font, new SolidBrush(this.ForeColor), textRect);
    }
  }

After building the project, look at the new component created in this example, the new control will be the MyGroupBox, so you will be able to access the BorderColor property:

myGroupBox1.BorderColor = Color.Red;