Well, you may try to use PageSetup.BlackAndWhite Boolean attribute to render the sheet in black and white. You got to loop through each sheet in the workbook to set the property to true before rendering to PDF:
e.g Sample code:
..........
//Get the first worksheet in the workbook.
Worksheet sheet = workbook.Worksheets[0];
sheet.PageSetup.BlackAndWhite = true;
..............