Watch the color explosion! You should be able to touch the swab several more times before the explosions stop. (If you want to try it again, you'll need to pour it out and start over.)
Why do the colors burst? The secret is in the tiny drop of dish soap. Have you ever wondered why cleaning a dish with soap is so much more effective than using just water? You can try this yourself by smearing some oil onto two identical plates. Give yourself just 30 seconds each to clean one with just water, and the second with water and dish soap. The one you clean with only water will probably still feel very oily, while the one you washed with soap feels much cleaner.
The reason soap is so good at cleaning things is because of a special property of the soap molecules. One end of each molecule is hydrophilic, or water-loving. That end of the molecule easily dissolves in water. The other end is hydrophobic, or water-fearing. That end won't go near the water molecules, but it will attach easily to the oil. With the hydrophobic end of the soap molecule grabbing onto the oil molecules, the plate is clean in no time!
That's exactly what you're seeing happen in the milk explosion. In addition to water, milk contains lots of fat and protein. Just like oil, the fat and protein molecules attach easily to soap molecules. When the soap touches the milk, it starts grabbing as many fat and protein molecules as it can. The attraction between the soap and the fat causes the molecules to move quickly, creating the colorful explosions you saw.