To conditionally format blank cells:
Select the data range that you want to format.
From the Home tab, click the Conditional Formatting dropdown and click New Rule.
Click Use a formula to decide what cells to format.
In the Format values where this formula is true box, type
=A2=”” (you can change A2 to be the first cell in the range to be formatted.)
Click Format, go to the Fill tab and choose an appropriate colour.
Click OK twice.