"Sorairo Days" roughly translates to "sky blue days."  This song was performed by Shouko Nakagawa and was used at the opening theme to the best anime (in my opinion) of 2007 "Tengen Toppa Gurren Lagann."  The story of Gurren Lagann is that humanity is forced to live underground by the Spiral King.  Our two main protagonists, Simon and Kamina, form Team Gurren with the goal of achieving the goal of visiting the surface world above which is soon realized with the finding of the Gunmen (mecha) named Lagann.  The overview of the story makes "Sorairo Days" fit the anime appropriately as it gives our protagonists a goal to see the outside world and make it so humanity can do the same.  The song itself is about looking forward towards creating your own future and not looking back to the past.

Without going into further details about the anime itself, it truly is a thrill to watch.  It has its fair share of epic moments.  It is one of the few series that I actually go back to and watch from time to time.  It really is that damn good.  I recommend that you watch it if you haven't done so yet.

Sora Iro Days English Lyrics