I am not a stickler for authenticity, but if you want an authentic Key Lime Pie it will be yellow instead of green.
Have you ever tried to juice tiny green Key limes without much success? That’s because green Key limes are just that -green, meaning they are not yet ripe! Key Limes are actually yellow when ripe. If you can’t find ripe Key limes and don’t have time for the ones from the store to ripen, go ahead and use regular (Peruvian) limes.
And if you are feeding 5 year old children who want their lime pie to be green, you will need to add a drop of blue dye, because even regular lime juice is too pale to make a green pie filling.