In a stand mixer with a whisk attachment, beat butter on high speed for 5 minutes - or until whipped and light in color. Scrape the bowl down throughout, as needed, with the mixer turned off.
Meanwhile, sift the powdered sugar (optional, but best results).
Reduce mixer speed to medium low, slowly add the sifted powdered sugar.
Beat until fully mixed, scraping the bowl as needed.
Add salt, vanilla extract; beat until fully incorporated.
Slowly add heavy cream until you reach desired consistency.
Separate icing and mix in colors, if desired. Using the stand mixer to mix the colors is easiest, but can also mix by hand.