Key limes are yellow, and that is the colour of an authentic Key lime pie, which is nothing more than a custard of Key lime juice, sweetened condensed milk and egg yolks in a cracker crust, then topped with meringue or whipped cream.
The show didn't exactly spell out the reveal (casual fans could have missed it), but there was one key to knowing the doe-eyed girl was the mother: the bright yellow umbrella she was holding.