Pour the warm milk into the bowl of your stand mixer. Stir in the yeast and 1 teaspoon of sugar.Let it sit for 5–10 minutes, until the mixture looks foamy.
Add the ¼ cup sugar, eggs, vanilla, and approximately 15 drops of purple liquid food coloring. Mix until combined.Add 4 cups of flour and the salt. Mix on low speed using the dough hook until a shaggy dough begins to form.
Add the softened butter gradually, about 1 tablespoon at a time, allowing it to mix into the dough.Add the remaining ½ cup of flour gradually. Knead on medium-low speed for 7–10 minutes, until the dough is smooth, elastic, and slightly tacky
Check the color after the dough is combined. Add more purple coloring a few drops at a time while kneading until it is slightly darker than you want. The purple will soften as the rolls bake.Place the dough in a lightly greased bowl. Cover and let it rise in a warm place for 1–1½ hours, or until doubled in size.