gistfile1.cs

var image = new Bitmap(100, 100, PixelFormat.Format24bppRgb);

using (var graphics = new Graphics(image))
{
	graphics.DrawRectangle(Colors.Red, 15, 15, 50, 50);
}

using (var locked = image.Lock()) unsafe
{
	int *value = (int*)locked.Data;
	var changePos = new Point(7, 7);

	value += changePos.X + (changePos.Y * locked.ScanWidth / 4);

	*value = Color.FromArgb(200, 0, 150).ToArgb();
}