private void Form1_Load( object sender, EventArgs e ) 
{ 
Bitmap newBmp = new Bitmap(100, 100); 
Graphics g = Graphics.FromImage(newBmp); 
g.FillRectangle(new SolidBrush(Color.Red), 0, 0, 33, 100); 
g.FillRectangle(new SolidBrush(Color.White), 34, 0, 33, 100); 
g.FillRectangle(new SolidBrush(Color.Blue), 68, 0, 33, 100); 
pictureBox1.Image = newBmp; //pictureBox1 was dropped on the form 
}

Contributed from George Shepherd's Windows Forms FAQ