How can I change the color of the gridlines of a Grid in WPF?

var T = Type.GetType("System.Windows.Controls.Grid+GridLinesRenderer," +
    " PresentationFramework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35");

var GLR = Activator.CreateInstance(T);
GLR.GetType().GetField("s_oddDashPen", BindingFlags.Static | BindingFlags.NonPublic).SetValue(GLR, new Pen(Brushes.Black, 1.0));
GLR.GetType().GetField("s_evenDashPen", BindingFlags.Static | BindingFlags.NonPublic).SetValue(GLR, new Pen(Brushes.Black, 1.0));

myGrid.ShowGridLines = true;