If using butter, beat 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. Skip this step if doing a dairy-free sugar-free buttercream.
Add the shortening to the stand mixer and mix until creamed with the butter.
Add in the Swerve and mix for about 30 seconds, or until just combined. Overmixing can lead to unwanted air bubbles.
Add in the vanilla extract and one tablespoon of whole milk (or water). Mix on low. If the frosting is thicker than a peanut butter consistency, add more liquid, a tablespoon at a time.
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.