In Serato 4.0, the third column in the library is the label color for that track. Click it to bring up a color palette, and select a color you wish to tag the file with.

For example: Party tunes could be purple, commercial tunes blue and instrumentals red - this can help you easily find the right songs to play at the right moment.

If you have colored a track, you will notice that the virtual deck will also change color when the file is loaded to the deck.