In the bowl of an electric mixer fitted with the paddle attachment, cream butter until smooth and creamy, 4-5 minutes.
Add one cup of powdered sugar, mix on medium speed until combined.
Add another cup of powered sugar, mix on medium speed until combined.
Add milk and vanilla; mix on medium speed until combined.
Add remaining two cups of powered sugar, one cup at a time, mixing after each addition.
Mix frosting on medium speed for 2 minutes.
Scrape down the bowl as needed, and check consistency of frosting. If it's too thin, add a little more powered sugar; if it's too thick, add a little more milk.
Once it's the right consistency, mix frosting on medium speed for another minute or two, until very light and fluffy.
Add coloring, if desired.