In a saucepan, whisk together the eggs and sugar until well combined. Whisk in the key lime juice, zest, and gel food coloring (I used the tiniest tip of a toothpick of gel, just to give the curd a subtle green tint. Completely optional. If you don't use, the curd will be a creamy yellow with flecks of green from the zest).
Turn the heat on to medium low and stir the mixture briskly and constantly until warmed through. Add the butter a little at a time until it melts into the mixture and becomes smooth. Continue cooking and stirring until the curd begins to thicken and coat the spoon. Pour the curd through a fine mesh sieve to strain. Pour curd into a jar or other tightly sealed container and refrigerate up to two weeks. Spread onto scones or biscuits or use as a filling for [desserts|http://alaskascratch.wpengine.com/2012/04/28/mini-lemon-meringues/] or donuts.