Попробуйте:
Option1:
ListBoxItem lbi1 = (ListBoxItem)(listBox.ItemContainerGenerator.ContainerFromIndex(0));
lbi1.Foreground = new SolidColorBrush(Color.FromArgb(100, 45, 23, 45));
Option2:
ListBoxItem lbi2 = (ListBoxItem)(listBox.ItemContainerGenerator.ContainerFromItem(listBox.SelectedItem));
lbi2.Foreground= new SolidColorBrush(Colors.Red);