Tracks that are either blue or gray in the library panel have been marked as played by Serato DJ. 

Click on the History button to open the History Panel, and click Reset This will revert all tracks back to white. 

You can also set Serato DJ to reset the played tracks everytime you exit the software, by checking the 'Reset Played Tracks on Exit' box, under the 'library + Display' tab in the setup menu.