To shade (gray) reference files in the background at print time, create a simple Pen Table and select the shade of gray (or any other color) you want to use. Then specify the pen table when you print.
From the Print Dialog, select Resymbolization (1) and create a New pen table.
Make sure you are on the Element Selection Criteria tab (2)
Select the Files (3) button and then the Select (4) button to choose the files to shade then Ok (5) to dismiss the Select Files dialog and Ok on the Modify Files dialog.
Now click on the Element Output Actions tab (6) and select Color (7) and choose the color the reference files should display (8).
Save the Pen Table (File - Save).
Now use the Pen Table when you Print the file.