I think the yellow is just because it's always been there, since the logo for Windows 3.1. Every few years, they slighly reinterpret the logo, removing the black borders between the colours, and now straightening out the swoosh. The rebranding campaign can't fully change the logo to something no one recognizes, so the colours were maintained. The logo now also looks more like four "tiles" used in Windows 8 and Windows Phone. Probably not a coincidence.
It just so happens they can make green into Xbox. I'm sure that wasn't the intention in the 80s when the Windows logo was first made.
Maybe they do have something up their sleeve for what yellow means, but I think it can only be speculated.