How do I highlight a field based on a comparison of another field?

There is no simple "Conditional formatting" like Excel has.  The only way to do this is to do a new sort of mirror field and maker a rich text formula field

https://help.quickbase.com/user-assistance/color_field.html

It's a lot less work simply to colorize the whole row of the report or else put up a warning icon beside the field.

But, your call.