Preheat your oven to 400 ℉ and line your baking sheets with parchment paper or a silicone baking mat.
Sift together the flour and baking soda in a mixing bowl and set it aside.
In a stand mixer, cream together the butter and sugar until it's light and fluffy. Add int he egg and vanilla and mix it together well. Then, slowly add in the flour mix and combine it into dough.
At this point, if you're making one flavor, add in the full Jello packet and any coloring you want. If you're dividing it, I found it easier to separate the dough, then add in the flavoring and color with hand kneading later. That way, you don't have to clean out the mixer bowl each time.
Load your cookie press with your dough with the shape you prefer, and press out your cookies onto your sheet. You don't need to leave a ton of space since they don't spread out, so you can get a lot on each sheet. Add any sprinkles you want here too.
Bake each batch for around 7-9 minutes, depending on whether you want crispier cookies, and based on how large the cookies are. In my pictures above, the red and blue cookies turned out softer, but the green ones were crispier. It's all tasty, so this is preference.
Let the cookies cool for about 5 minutes on the baking sheet, then move to a cooling rack to finish cooling.