C#

     R
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   *****
G ******* B

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
 
class Test : Form
{
  static void Main()
  {
    Application.Run(new Test());
  }
 
  Test()
  {
    Text = "Triangle Example";
 
    Triangle t = new Triangle();
    Controls.Add(t);
    t.Dock = DockStyle.Fill;
  }
}
 
class Triangle : Control
{
  public Triangle()
  {
    SetStyle(ControlStyles.UserPaint | ControlStyles.AllPaintingInWmPaint
      | ControlStyles.DoubleBuffer | ControlStyles.ResizeRedraw, true);
 
    UpdatePath();
  }
 
  protected override void OnPaint(PaintEventArgs e)
  {
    base.OnPaint(e);
 
    if (!init)
    {
      UpdateBrush();
      init = true;
    }
 
    Graphics g = e.Graphics;
    g.Clip = region;
    g.FillPath(brush, path);
  }
 
  protected override void OnResize(EventArgs e)
  {
    base.OnResize(e);
    UpdatePath();
  }
 
  protected override void Dispose(bool disposing)
  {
    if (disposing)
    {
      if (path != null) path.Dispose();
      if (region != null) region.Dispose();
      if (brush != null) brush.Dispose();
    }
  }
 
  GraphicsPath path;
  Region region;
  void UpdatePath()
  {
    PointF p1 = new PointF(Width / 2, Top);
    PointF p2 = new PointF(0, Bottom);
    PointF p3 = new PointF(Width, Bottom);
 
    if (path != null)
    {
      path.Dispose();
      path = null;
    }
 
    if (region != null)
    {
      region.Dispose();
      region = null;
    }
 
    path = new GraphicsPath();
    path.AddPolygon(new PointF[] {p1, p2, p3});
    region = new Region(path);
 
    if (init) UpdateBrush();
  }
 
  PathGradientBrush brush;
  bool init = false;
  void UpdateBrush()
  {
    if (brush != null)
    {
      brush.Dispose();
      brush = null;
    }
 
    brush = new PathGradientBrush(path);
    brush.CenterColor = Color.Gray;
    brush.SurroundColors = new Color[] {c1, c2, c3};
  }
 
  Color c1 = Color.Red, c2 = Color.Green, c3 = Color.Blue;
  public Color C1
  {
    get { return c1; }
    set { c1 = value; UpdateBrush(); }
  }
 
  public Color C2
  {
    get { return c2; }
    set { c2 = value; UpdateBrush(); }
  }
 
  public Color C3
  {
    get { return c3; }
    set { c3 = value; UpdateBrush(); }
  }
}