If you want a drink that's NOT a McDonald's Shamrock Shake, but is just as unnaturally green, here you go.
A website called Totally The Bomb just revealed how to order a "Shamrock Tea" off the Starbucks secret menu. Which, by the way, doesn't really exist so you'll need to give these directions explicitly.
Order an Iced Venti Green Tea Latte with coconut milk . . . add a splash of lemonade . . . and add two scoops of vanilla bean powder. Then enjoy whatever all THAT tastes like.
(Here's a picture.)