public delegate void ChangeColorHandler(Color color);
public event ChangeColorHandler ChangeColor;

private void button1_Click(object sender, System.EventArgs e)
{
	ColorDialog cd = new ColorDialog();
	if(DialogResult.OK == cd.ShowDialog())
	{
		ChangeColor(cd.Color);
	}
}

Form2 f = new Form2();
f.ChangeColor += new colorgrid.Form2.ChangeColorHandler(f_ChangeColor);
f.Show();
............

private void f_ChangeColor(Color color)
{	 //обработка 
	this.BackColor = color;
}