Well a simple solution for you would be to alter the color's opacity by setting the color's alpha value to something lower than 255. Example:
Code:
using System.Drawing;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void Form1_Paint(object sender, PaintEventArgs e)
{
Graphics g = e.Graphics;
g.FillPolygon(new SolidBrush(Color2Transparent(Color.Red)), new Point[] {
new Point(10, 10),
new Point(100, 100),
new Point(10, 110)
});
g.FillPolygon(new SolidBrush(Color2Transparent(Color.Blue)), new Point[] {
new Point(50, 10),
new Point(150, 100),
new Point(60, 110)
});
}
private Color Color2Transparent(Color c)
{
return Color.FromArgb(128, c.R, c.G, c.B); // setting opacity to about 50%
}
}
}
The result will look like this: http://i45.tinypic.com/9lj3hx.jpg