I think that's the formal Resistance pilot uniform, during the movie you can see some of the other pilots wearing it before they suit up in the orange one and get on their planes. Or maybe it's a commander uniform.

Here's Admiral Statura's uniform: http://www.theforce.net/photo.asp?image=http://www.theforce.net/2015/kltfa.jpg
I think what Poe's wearing is something similar to that.