In a mixing bowl, combine milk, vanilla extract, and salt. Afterward, stir the ingredients together to dissolve the salt, then set it aside.
After that, attach your stand mixer with a paddle attachment, then place the butter inside the bowl. And beat it at medium speed for two minutes until it becomes lighter in color and creamy. Next, scrape off the bowl's bottom and sides with a rubber spatula.
Afterward, add the milk mixture. Then, add the shortening. With that, make sure to use a splatter guard over the bowl to prevent splatters. You can also use a towel over the top of your mixer if the splatter guard is unavailable.
Then, make sure to set your mixer to low until the liquid begins to incorporate. And after that, turn it to medium for twenty to thirty seconds until every ingredient is smooth and combines well. Afterward, scrape down the bowl to properly combine.
After that, add the sugar to the mixing bowl. Ensure to do it at a steady flow on a small opening of the splatter guard or towel. Because of this, if all of the sugar incorporates well, scrape down the bottom and sides of the bowl. Then, turn the speed of the mixer to medium-low for about thirty seconds to mix the frosting properly.
Equally, divide your frosting into six bowls. Then, add a different gel coloring into each bowl and mix well. In that process, you can add more if needed to achieve the desired color of your choice. Afterward, place each color of frosting into individual piping bags. After that, set the piping bags aside.
Next, take your stacked, crumb-coated, and chilled cake. And then place it into a turntable. After that, cut an opening for each piping bag with scissors. Afterward, cut the opening identically as possible.
Then, begin by piping layers of multi-colored frosting from the bottom of the cake. After covering the bottom of the cake, start piping on the top starting from the outside edge of the circle.
After that, smooth and spread the top and sides of the cake by using an offset spatula. In addition to that, you can use a bench scraper if you desire. Then, wipe the frosting off the spatula each time you use it to avoid the colors from mixing.
After smoothing the cake's frosting, you can pipe swirls on top of your cake. Likewise, you can add sprinkles. Then, chill your cake for an hour until the frosting is firm.
Lastly, take your chilled cake. Then, cut the cake and enjoy!