What do zones mean?
I have the price alerts set up. Every time price reaches a level, I get a notification (phone & e-mail). Depending on the budget left to invest, I decide where my cash goes to. For example, if I still have a lot of money to invest (monthly budget), I can be willing to buy stocks at higher price levels (sometimes even above red zone). If I run out of money for the month, I am waiting for price to go lower, as I have not enough funds to buy more stocks at the same time.
To sum up, the more money I have, the higher price can be to purchase a stock as I still can buy more companies.
Generally, if any stock reaches the bootom of the green zone, I expand my budged to make an instant purchase.