I had not paid attention to this, but it would seem ambientColor no longer works.
On this PG I put a green atmosphere, but it does nothing.
http://www.babylonjs-playground.com/#1SLLOJ#296
By cons if I remove the sky, it works. the scene is in a green atmosphere as can be seen see on this PG
http://www.babylonjs-playground.com/#1SLLOJ#297