In a large mixing bowl, beat the sugar, powdered sugar, and butter together using a hand mixer for 3–4 minutes until light, creamy, and fluffy.

Add the vegetable oil and egg, continuing to mix until fully combined. The mixture should be smooth and slightly thick.

In a separate bowl, whisk together the flour, salt, baking soda, and cream of tartar. Slowly add the dry ingredients to the wet ingredients one cup at a time, mixing on low-medium speed. You may need to use your hands to finish combining everything if the dough becomes too thick for the mixer.

Divide the dough evenly into three bowls. In two small bowls, mix 1/2 tsp cherry and berry Jello powder with 1/2 tsp water each to create colorful liquid flavorings.

Pour the cherry mixture and red food coloring into one dough portion and knead until fully red. Repeat with the berry mixture and blue food coloring in another dough portion. Leave the third portion plain.

On a parchment-lined tray, shape the plain dough into a long, even block (about 8–10 inches). Shape and press the red dough alongside one side and the blue dough on the opposite side to form a tricolor log.

Cover tightly with plastic wrap and chill for 30 minutes to help the dough firm up, making it easier to slice.

Preheat the oven to 350°F. Slice the chilled dough into 1/4-inch crosswise cookies. Place each cookie on a parchment-lined baking sheet, placing a popsicle stick underneath or directly into the dough. Lightly press each cookie with a fork to add decorative lines.

Bake for 10–14 minutes or until the edges just begin to turn golden and the centers are set. Cool completely before serving.