Step One: Mix Powdered Sugar and Butter
In a large bowl, mix the powdered sugar and butter with an electric mixer on low speed until fully combined. You may also use a wooden spoon with you do not have an electric mixer.
Step Two: Add Vanilla and Milk
Add the vanilla and one tablespoon of milk to the mixture. Mix until fully combined.
Step Three: Add in Remaining Milk
Gradually beat in remaining tablespoon of milk until the frosting has reached a smooth and spreadable consistency. If the frosting is too thick, you may add a few drops of milk at a time until it reaches a creamy consistency. If the frosting is too runny, you may add a small amount of powdered sugar until it thickens up.
Step Four: Add the Food Coloring
Once the frosting is fully mixed, add the food coloring. It will take approximately 60-70 drops of navy blue food coloring for four cups of frosting to reach the desired shade.
Thoroughly mix the food coloring in the frosting until it is fully mixed. You may use an electric mixer, wooden spoon or spatula to mix the food coloring into the frosting. You may add more food coloring if it does not come out of your desired shade.
If you are using store-bought frosting, use a spatula to scrape the frosting into a bowl. From there, you can add food coloring to the frosting and thoroughly mix together.