Place butter and shortening in mixer and cream together on medium speed. Add in vanilla and salt and continue mixing.
Begin to add confectioners sugar one cup at a time, mixing on low speed until combined then med-high speed for a minute or so each time. After each cup addition, add a tablespoon of water to get your desired consistency. Add gel food coloring and fully incorporate it.
Notes
Buttercream frosting can be made ahead of time and stored in an airtight container in the fridge for up to 3 days before using.
If you have leftover frosting, you can freeze it in a sealed container for up to 3 months.