Pour water and key lime juice into a small saucepan. Sprinkle gelatin over liquid and let it sit for 5 minutes. Add sugar and salt. Place pan over medium low heat and stir until gelatin and sugar are completely dissolved. Remove from heat and let cool slightly.
Pour gelatin mixture into a medium sized mixing bowl. Whisk in the yogurt and one half of the container of whipped topping until smooth. Stir in food coloring, if desired. Pour into crust and refrigerate for 4 hours or overnight.
Serve with more whipped topping, grated lime zest, lime slices and cherries on top.
Store covered in refrigerator for up to 3 days.
Video
Notes
I recommend using fresh key limes or bottled key lime juice. If you can't find either, go ahead and use regular fresh limes or bottled regular lime juice.