In a mixing bowl, beat together the butter and cream cheese until combined (about 25 - 30 seconds), then add powdered sugar and salt. Beat until light and fluffy, 3 - 5 minutes.
Beat in the vanilla and coloring until no streaks remain.
Use immediately or store in the refrigerator for up to 1 week. If the frosting is too soft to pipe right away, refrigerate in 15 minute increments until piping consistency.
Notes
You can add a variety of food coloring for different colors and you can use different extracts for bold flavors [Orange, Banana, Lemon, etc.]. Great frosting to use for decorating pastries!