Look at Conditional Formatting

Compare Rental Amount vs Paid amount and set cell background if not equal

With Rental value in A1, Payment in B1 then set CF for B1 as

Formula is: =B1<A1 and set pattern colour

HTH

"Blue Eyes in South Ga." wrote:

I have a set rental amount that the customer pays, but they may pay part of
that, I want to format the cells in excel to show that the full payment was
not made. Anyone know how????