Making purple frosting isn't difficult and can be made with the simplest of supplies using plain white frosting, homemade or store-bought. It doesn't take much time or effort to make the exact color you need. Add your colors slowly and you'll get exactly the right shade.
Take 1/2 cup (or one stick) of butter of room temperature butter and place it an bowl and beat with an electric mixer until fluffy.
Add in 1/2 tsp. of vanilla extract with 2 tbsp. milk. Beat on high until blended. Add more milk if your frosting is too stiff, one tablespoon at a time.
Place 1 cup of powdered sugar into the butter mixture and beat until blended. Repeat until all the powdered sugar is gone.
Beat until fluffy and then add 10 drops of red food coloring for every five drops of blue coloring. Blend. Add color until you achieve the hue of color you'd like.