Prepare the baking pan by greasing it lightly with butter or lining it with parchment paper to prevent sticking.
In a large saucepan, melt the butter over low heat. Once melted, add the marshmallows and stir constantly until completely melted and smooth.
If desired, add a few drops of food coloring to the melted marshmallow mixture. Stir until fully combined.
Remove the saucepan from heat and add the vanilla extract, mixing well.
Pour the Rice Krispies cereal into the marshmallow mixture and stir until all the cereal is well coated.
Gently fold in the pastel candies, reserving a few for topping the treats later.
Transfer the mixture to the prepared baking pan. Using a spatula or your hands, press the mixture firmly and evenly into the pan.
Sprinkle the reserved candies on top and gently press them into the mixture.
Allow the treats to cool at room temperature for about 30 minutes before cutting them into squares or rectangles.