If checkbox is no/false highlight row?

I am trying to highlight a row if the Checkbox field "Confirmed Sent" is false/No.

We have 2 ways to send out invoices, send now or sent via email. I have 2 fields that are checkboxes, the first is "Send Now" and the other is "Sent via Email?".  I created the 3rd "Confirmed Sent" which looks at both these fields to confirm the invoice has been sent either way. 

Now I want to highlight any row where "Confirmed Sent" is not yes/true.  

I tried using the IsNull function in the report settings with no luck:  

If(IsNull([Confirmed Sent], "yellow"))

I also tried creating a formula field to do the same but I keep getting syntax errors.  

Any ideas???