Cream cheese and butter should be mixed well until they are smooth. A stand mixer is what I like to use for this, but a hand mixer will do.
Add only one cup of powdered sugar to the cream cheese mixture. Then add the salt and peppermint extract. Make sure that everything is mixed evenly by scraping down the sides of the bowl.
Mix in the remaining 3 cups of powdered sugar one large spoonful at a time. Avoid adding sugar all at once because it will be crumbly and hard to mix. After adding all the sugar, scrape the bowl again and beat for 1 minute.
Add gel food colouring if using. Divide the mint mixture evenly into mixing bowls based on the number of colours you'll make. You can hand-mix the food colouring into the mint mixture(s).
Place teaspoon-sized balls, starting with the first colour, on baking sheets lined with parchment paper. Use your hands to shape each one into a smooth ball.
The next step is to dip a fork in powdered sugar and make a hole in each mint with the tines of the fork. Do it again with the last colour.
Dry the mints in the air for two hours before putting them in the fridge. You can turn your mints over and let them dry a little longer if the bottoms are still soft.