Conditional Format on 2nd drop down menu - Invalid option

I have two drop down menus, the 2nd menu is an indirect formula on the 1st menu. Let's say Drop down 1 is selected and then 2 is selected, then the user decides to change drop down 1, the option in 2 is no longer valid.

How do I do a conditional format on drop down 2?

This is what I currently have but does not work:
Conditional Format in F16; =indirect($D$16)<>$F$16
D16 = Drop down 1
F16 = Drop down 2

*I already have a VBA which auto clears out the 2nd drop down if the 1st one is changed, however not every user has the newest Excel*

Thanks

You could conditionally format the second dropdown to turn red if it needs to be updated.

What versions of Excel are others using that your macro doesn't work for them?

Anyway, here's an example. with the first dropdown in A1 and the second (dependent) dropdown in B1, I set this Conditional Formatting

=ISERROR(VLOOKUP($B$1, INDIRECT($A$1),1,FALSE))
So if it can't find the value in B1 on the list in A1, it will change color.

Makes sense. Perfect. Thank you