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

Click on the setup cog in the top right hand corner of your screen and head to the 'Library + Display' tab. From here, you can click the 'Reset Played Tracks' button to reset played tracks to white.

You can also set Serato DJ to reset the played tracks every time you exit the software, by checking the 'Reset Played Tracks on Exit' box, shown below: