Add 2 scoops vanilla ice cream and 1/4 cup milk to blender, then blend until smooth. Add more milk or ice cream, if needed, for desired consistency.
Divide milkshake mixture evenly between 6 cups.
Add a few drops of red food coloring to the milkshake mixture in one cup and stir until it turns red. Add more red food color and stir again, if needed, for desired color saturation. Repeat, using a different color of food coloring for each cup of milkshake mixture.
Pour the colored milkshake mixtures into layers in a milkshake glass, in rainbow order: red, orange, yellow, green, blue, and purple.
Add some whipped cream on top, then decorate with gummy bears and rainbow sprinkles.