In a medium bowl, empty contents of pudding mix and cold milk. Whisk.
Using the green gel food coloring, add in 2-3 drops. Whisk everything together for 2 minutes.
Set it aside for 5 mins to set.
Mix half the frosting with brown food coloring until it is fully mixed in.
Add a few drops of black food coloring and mix in.
Add the other half of the frosting back into the black colored frosting and mix well to fully incorporate the color throughout.
Put the frosting into a piping bag fitted with a 1M tip. Add the green pudding mix into a separate piping bag and cut off the bottom.
Cut the centers out of the cupcakes with a paring knife and add the slime (pudding) to each one.
Put the top back on each cupcake and gently press down on each one.
Pipe black frosting onto the top of each cupcake and add sprinkles.
Notes
The time doesn't include time for making the cupcakes. You can use either premade and undecorated cupcakes or homemade ones.Make sure to use the GEL food coloring for best results for the color.