Conditional shading without VBA

Ok, First off, I'm no expert at Excel, so be lenient if this is a bonehead question.

What I need to do is check the Date column for an invoice. If the date column is empty, I need to leave the checkbox column blank. If there is something in the Date column, I need to shade the checkbox so that when its printed out, we know the invoice was received.

What I did is put this formula in the checkbox column:

if (c2="","", "##")

This puts two pound symbols in the checkbox if there is an invoice number in C2, but that's not the way it should be done. How would I, if I can, change this formula to shade the cell instead of putting "##" in it?