Beat the room temperature butter on medium-high speed for about two minutes, until it is creamy and lighter in color. The whiter the butter is, the whiter the resulting buttercream.
Mix in the Sweetex hi ratio shortening on medium speed until fully creamed into the butter, about one minute.
Add in the powdered sugar and use the rubber spatula to mix in the sugar a bit before turning on the mixer to low. This helps prevent a kitchen covered in powdered sugar. Mix for about 30 seconds, or until just combined.
Add in the teaspoon of pure vanilla extract and one tablespoon of whole milk. Mix on low. I prefer a consistency of smooth peanut butter for my buttercream icing. Add in more milk as needed until you achieve your preferred consistency, keeping in mind that too much milk will lead to less crusting.
Separate the frosting into individual bowls for each color of icing. Add in the preferred gel colors, using a fork to mix. Place into prepared icing bags.