Hey there, 

I'm looking for this fic, Cas was a fledgling angel and crashes into Gabriel Heaven and Cas ends up helping Gabriel deliver some messages, one to Mary (I think), I can't remember the other two but near the end Cas gets separated from Gabe and tries to finish sending the message himself and it starts to drain his grace or something. I remember they had this play fight somewhere with coloured dust or something and Kali was in that scene. It was part of a series, with all the other angels looking after Cas or playing with him or whatever. If anyone remembers it, please link me!