Don't laugh. I just got a note from an umpire prospect in response to a note I sent where we require Black shirts for a particular tournament. He wanted to know if a regular black polo shirt would be OK, because he doesn't have a black-colored umpire shirt.
As for pants, I bet you can get some inexpensive heather gray dress ones from your local St. Vincent de Paul or Salvation Army store. They have their 25-cent sales every now and then.
Jerry