Move to the cell on the worksheet where you want to start conditional
formatting.
Click FORMAT in the menu and select CONDITIONAL FORMATTING.
In the CONDITION1 combo box, chage VALUE IS to FORMULA IS.
To the right of the combo box enter the following formula, substituting A1
with the cell address you're currently assigning the format to:
=A1=""
Click the FORMAT command button and click the PATTERNS tab and select the
color you want if the cell is empty. Click the OK command button 2 times to
exit. then copy cell A1 down the column as far as necessary.