It probably depends on how the food coloring likes acid--but easter egg coloring uses vinegar to color the shells of eggs--just a thought.
Whatever you use, if natural it would need to stay green in an acid solution, so if Heinz could make Green Ketchup, with spinach, maybe spinach would work?
Sprinkle chlorella, spirulina, or a more suitably flavored green product into the recipe. Might I recommend parsley, especially stale, finely ground parsley?
- See more at: How To Make Green Food coloring
Good luck, I'd like to see how they worked out for you!