for 2007, 2010 , 2013 or 2016 excel version
Conditional Formatting
Highlight applicable range >>
B2:af5000
Home Tab >> Styles >> Conditional Formatting
New Rule >> Use a formula to determine which cells to format
Edit the Rule Description: Format values where this formula is true:
=and($A2<>"",B2="")
Format… [Number, Font, Border, Fill]
choose the format you would like to apply when the condition is true
OK >> OK