These were so fun to make (thanks Dan and Julia!) and they look spectacular AND they are delicious!
Just a few tips:
1) It's not as hard as it looks to beat egg yolks to stiffness -- if you have a stand mixer! But even a little yoke in the egg whites can cause problems.
1) You will know the egg whites have formed stiff peaks if they hold their shape when upside down
2) You must use food coloring gel...not just plain food coloring.
3) Maybe use a lighter coating of gel if you want a more pastel cookie. OR buy pastel colored food gels. I only had primary colored gels. For more effect, you could even dye the meringue before painting the rainbow.
4) Filling piping bag was tricky - but turned out ok - don't be nervous about this step
Here's the recipe!