Make Cloud Jars: Cut cloud shapes out of duct tape and press firmly onto mason jars. Paint light blue craft paint onto jar and let dry before removing tape.
Make Rainbow Cookie Template: Start by making a rainbow template by using a round cookie cutter and card stock. What size cutter you use will depend on the size of your jar. Before cutting, hold the paper up to the top of the jar and cut across the circle at a point that would work for a rainbow. Cut out notches and set it on the jar to make sure it stays put.
Make royal icingand divide up into six portions and add a different food coloring to each. Cover with plastic wrap and set aside.
Roll out the dough to about 3/8 inch thick. You want it on the thicker side so that it will stay up on the rim of the jar.
Lay the template on the dough and cut out rainbows with a small, sharp knife. Transfer them to baking sheet.
Place the baking sheet into the freezer for about 15 minutes before baking to help the cookies to hold their shape.
Bake the cookies according to recipe directions, or about 18 minutes at 350 degrees.
Allow the cookies to cool completely and firm up. Take one of the cookies and test it on a jar. Use a small, sharp knife to gently scrape away from the edges of the notches (the parts that will rest on the rim of the jar) until they are sharp and even and stay firmly on the jar. (You can even gently cut into the baked cookie before it is completely cooled and get good results.)
Fill the decorating bags with the royal icing, fitted with #3 tips. The consistency of the icing should be thick enough to stay put as you apply it to the cookie, but not too stiff.
Apply every other rainbow color to start (see photo above) and let dry for about 20 minutes. Apply the other icing, filling as you go.
Let cookies dry completely, add milk to your jars and place the rainbow cookies onto your jars. Add a straw and you're ready for the perfect rainbow party treat!