Whilst enjoying the company of Twilight Sparkle and her friends in the doughnut shop immediately following the events of The Best Night Ever a disappointed Princess Celestia send a letter off to Prince Blueblood explaining the implications of his failure of the "Rarity Test", among other things...

When your life is as dull a gray as the world that surrounds you, the mundanities can make it all seem meaningless. Sometimes all we need is a little color -- or six -- to reintroduce us to what truly makes life worth living.

*6-16-2017 Edit, added Rainbow Dash tag because it triggered knighty.*