Spray a pot with baking spray, melt butter, add in marshmallows and stir over medium heat until melted.
Remove from heat and stir in Fruity Pebbles.
As soon as they are cool enough press them into your mold halves, be generous. The smush it together until they click or place a rubber band around it to hold shut. Allow them to cool and firm up. Place in cold garage or fridge to speed up the process.
Unmold and serve when ready.
You could press candy on top to mimic the top of a Christmas ball.
Notes
If you do not have molds, you can use food safe gloves and pack like a snowball. Nice and firm. Place on cooling rack to cool.