Grid Background is not changing

Question

I have to set the grid background based on the result. that is working fine.

 

But when I am clicking again the button, it is not set to white color. 

private void Conversion_Click(object sender, RoutedEventArgs e)
{
   grid.Background = new SolidColorBrush(Colors.White);
   bool result = DoConversion();

   if (result)
   {
         grid.Background = new SolidColorBrush(Colors.Green);
   }
   else
   {
         grid.Background = new SolidColorBrush(Colors.Red);
   }
}