Gently mix all frosting ingredients until smooth. A stand mixer is the easiest way, but a whisk will work if your butter is soft enough. I warmed the butter up in the microwave for about 10 seconds the last time I made these.
If you want to color the frosting, you can divide it into batches to color. Add a few drops and mix it up until they are as bright as you like.
This frosting sets up at room temperature, but if it is warm, we put it in the refrigerator for a few minutes.