Do you have the bill "amount due" and the "amount paid" in separate
columns? Let's say you have the bill name in column A, the "amount
due" in column B, and the "amount paid" in column C. You can use
conditional formatting in all three columns to highlight the cells
that you want.
1. Go to column A (we'll use row 1 for example) and go to conditional
formatting
2. Change the drop-down, if using prior to Excel 2007, and change the
value to "formula is"
3. Input the formula like =if($c1=$b1,true, false)
4. Click the "format" (I think, I'm using excel 2007, and am trying to
write this off memory) box to change the format to the desired result,
for example change the fill color.
5. click enter
Let me know if this works!

Andrew Armstrong