I remember one fic actually gave a rather good in-universe explanation as to why Celestia was Princess and not Queen, and that was that, for a long time, Luna had been the decided heir to the throne. After the incident 1000 years ago, Celestia had become active ruler, but refused to accept the tittle of Queen, saving it for her sister's eventual coronation.
That whole deal aside, well... this is a really cute pic.
"A World Without Rainbows", it's here in DA. It's a bit dark, but nothing too much. It's got a very interesting plot, but the bit I mentioned regarding Celestia and Luna is really only a detail that's irrelevant to the plot.