Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper. Whisk flour, baking soda, and salt in a medium bowl. Set aside.
Beat softened butter, granulated sugar, and brown sugar until light and fluffy (2–3 minutes).
Add eggs one at a time, mixing after each. Stir in vanilla extract. Gradually add dry ingredients to the wet mixture. Mix until just combined.
Split dough into four portions. Color each with a different pastel gel food coloring (pink, purple, yellow, blue). Mix gently.
Press small portions of each colored dough together. Roll into a ball for a marbled effect.
Place dough balls 2 inches apart on the baking sheet. Flatten lightly and add extra chocolate chips on top. Bake for 10–12 minutes until golden at the edges.
Let cookies cool on the sheet for 5 minutes, then transfer to a wire rack to cool completely.