Board Thread:Fleet's Prepared-ness/@comment-30268812-20161114023045/@comment-27862274-20161115124957

http://kancolle.wikia.com/wiki/Events/Mechanics

A ship tagged with a specific colour tag cannot be sortied in other coloured maps. A tag is automatically stuck on your ship the moment you sortie into a map, and is impossible to remove. For example, a ship that gets a Red tag in E1 cannot be sortied in other maps with different colours, but can be deployed in a map with the same Red tag (let's say E3) - limiting where your ship can be sortied. This means you must carefully decide what ships you use - because they cannot be used in other coloured maps once they are tagged with a specific colour. It also forces you to have a larger number of well levelled ships.

Honestly though, i would HIGHLY advise you not to try medium, because your ships simply aren't ready - maybe even for easy - you don't have enough ships or levels. Shiplock only applies for Medium/Hard not for easy difficulty - so if you do easy mode you can completely ignore it.