Heat oven to 200 degrees F.

Line two sheet pans with parchment paper or top with silicon baking mats.

Prepare a pastry bag and fit it with a large plain / round tip. (If you don't have a pastry bag, just use a large zip-top bag with one corner cut off.)

If not already separated, crack eggs and separate whites and yolks. (Note: Do this carefully. If there are any yolks mixed in with the whites, they will not whip correctly.)

In the bowl of a standing mixer or with a hand mixer, beat the egg whites on medium-high until light and foamy, ~2 minutes. Turn the mixer down and slowly add sugar while the mixer is running. Add cream of tarter.

Increase mixer to high and continue mixing until egg whites are smooth and glossy, ~5 minutes more. (Note: You'll know this is done when the mixture forms "stiff peaks" and stays in those peaks if you turn the beater upside down.)

Add food coloring / gel, mixing on medium, until the meringue reaches your desired color of green.

Transfer the meringue to a piping bag and pipe into tree shapes on prepared baking sheet, making the base of each tree about the size of a quarter.

Top the trees with sprinkles.

Transfer trees to the oven and bake until very dry and crisp, rotating the pans halfway through, 2 to 2 1/2 hours. Test for doneness by pulling one of the trees off the parchment paper. If it releases easily and is very crunchy, it's done. (If the trees show any signs of turning golden brown before they are crisp, reduce heat to 175 degrees F.)

Store in an airtight container at room temperature. Will keep for 3 to 5 days.