Mix the condensed milk, limeade, and Cool Whip with a mixer. If you don't have or don't use food coloring often, then you can skip it. The color is just kind of anemic if you don't add the coloring. Pour this in the graham cracker crust. Chill for an hour or more. An interesting and good variation on this is to replace the limeade with pink lemonade. Do not add green coloring in that case. But 1 tsp of red which will give a pink color. This pie gets raves while being super simple to make. It's a good recipe to turn kids on to cooking. It's also fairly low in cost. To further cut costs, make your own graham cracker crust by mixing about 1 1/2 cups graham cracker crumbs, 1/2 stick melted butter, and 2 TB sugar and pressing that out in your own pie pan. If you don't keep graham crackers (or the bagged crumbs) on hand, then you're probably better off to go ahead and buy the crust in the throw away pan. Note — The standard 9 inch graham cracker crust will not quite hold all the key lime mix. I like one bigger pie over the 2 thinner pies. Another option is to get the small pie and then the little 4 pack of individual pies and have divide it that way.