In an estimate your paper will turn red if your paper price has expired. It can be expired in two different was. 

1. The first is the pricing experation date in the paper record on the pricing tab. 
2. The second is the price mod date on the same screen. The number of days until a price is expired is set in your preferences (as shown below). If the price mod date is older than the number of days noted in your preference then the paper will be red.