Help! With finding a formula!

I created a monthly bill sheet for work, that has colums for every friday to keep track of bills. On the top row, It adds together the total amount due for each week. Below that row, I am trying to add a "Paid" row that will subtract what is paid from what is due to always have an updated amount due. By either turning the cell green, or bolding the text, something to help us remember that it is paid. & then have the numbers automatically update in the row..