TfL have announced official names and colours for the Overground lines:
We’re giving the lines on the London Overground names that celebrate London’s wonderful and varied cultural heritage. In this blog post, we tell you all about the names, the stories behind them and when the changes will come into effect.
Apparently it’s the Liberty Line because the “we’ve spent far too much time trying to find something nice to say about Romford” Line wouldn’t fit on the signs.