Line an 8x8 baking pan with parchment paper and spray with nonstick cooking spray.
Add butter, chocolate chips and marshmallows to a very large saucepan and heat over medium low heat, stirring constantly until mixture is completely melted. This takes some time, about 10-12 minutes. Remove from heat and fold in food coloring. Mix until evenly colored. Cool for 2 minutes. Fold in gumdrops.
Spread mixture into prepared pan. Add reserved gumdrops to the top and gently press into the candy.
Let stand to cool completely and loosely cover, let stand overnight.
The next day, remove from the pan and discard the parchment paper. Use a sharp knife to cut into 30 squares. Serve immediately or cover until ready to serve.