In a 2 cup measuring cup, add warm water and Fruity Pebbles. Let steep for 20 minutes.
Prepare white cake mix in a 9×13 pan according to box instructions. Add in 1 cup Fruity Pebbles and bake as directed.
While the cake is baking, prepare the simple syrup by straining the steeped cereal through a fine-mesh sieve into a sauce pan. Allow to sit for several minutes to allow all liquid to drain. *Do not squish mushy cereal through the sieve*
Add sugar to the pan with the cereal water. Over medium-high heat, bring to a boil. Boil for 5 minutes.
Stir in food coloring. Set aside until ready to assemble cake.
While the cake is still warm, use the end of a wooden spoon to poke holes all over the surface of the cake.
Pour sweetened condensed milk onto cake, using a spatula to spread it out and help it get into the holes.
Carefully pour the simple syrup evenly over the cake into the holes.
Spread whipped topping over top of the cake.
Top with Fruity Pebbles (Note: Do not top with the Fruity Pebbles until you are ready to serve, as they will become soggy.) Serve immediately, or store in the refrigerator until ready to serve.