Combine sweetened condensed milk with unsalted butter in a wide, no stick pan over medium heat. Cook, stirring constantly, until obtaining a homogeneous mixture (without lumps).
Mixture is ready when it thickens and starts to peel away or show the bottom of the pan when you scrape it with your wood spoon (about 10-15 minutes, depending on how potent your stovetop is).
NOTE: The fudge mixture should be thick enough to reveal the bottom of the pan for a couple of seconds before the mixture levels out again.
When the fudge mixture is ready, remove from the stovetop and stir in the vanilla extract. Next, spread 1/3 of the mixture in the center of a large plate greased with unsalted butter.
Place another 1/3 of the mixture in a bowl and stir in about 15 drops of blue food coloring and 2 drops of red. Then, place the remaining 1/3 of the fudge mixture in another bowl and stir in about 15 drops of red food coloring and 2 of blue.
Transfer both the blue and red mixtures to the greased plate, spreading them around the white as shown in one of the pictures above. Let cool completely!
Using a tablespoon as measurement, scoop out one spoon of the white/red/blue mixture (starting from the center to the edge of the plate) and with greased hands, roll out into balls.
Place each bowl into a paper bonbon cup and if desired, place a few patriotic color star-sprinkles on top. Enjoy!