Add the softened butter to the bowl of a stand mixer and beat with the paddle attachment until smooth and creamy.
Add the powdered sugar, milk, vanilla extract, peppermint extract, and salt to the bowl. Beat to combine.
If adding food coloring, divide the dough into sections and wrap each section in plastic wrap to keep it from drying out.
Place one portion on a surface dusted with powdered sugar. Add one to two drops of food coloring and work the coloring into the dough. Make sure to wear plastic gloves to keep the food coloring from staining your hands.
After the coloring is fully incorporated, roll out the dough into a long rope and cut one-inch pieces with a knife.
Transfer the pieces to a parchment paper lined baking sheet and allow them to sit at room temperature overnight to dry out.
Repeat the process with each color until all the dough has been used.
Once mints are dry, store in an airtight container.