Well, the only way I know at the moment to set the white color on White Ambiance bulbs is through the Hue app itself. Setting a Home scene would only ask for the light to (in this case) be on. It will come on at whatever color you were at last time in my experience. The temperature in the Hue app is called Read for @2700k. Relax is lower, something like 2100 or 1800, I forget but you can set a scene there if you like. That is all I know to do at the moment. Like I said earlier, I have my lights, through the Hue app routines, set themselves to Read 5 minutes before my Home app turns them off. That way each time I ask for them to be on they will be @2700.
I also have the 4 button accessory with scenes available by pushing the on button until you reach the color you like. We normally use 1 push for 2700, 2 pushes for the much warmer and dimmer Relax and do not use the others.
How do you know the "Read" scene is 2700K / warm white like an old incandescent light? I have my default one as "Bright", so is that not 2700K / warm white? Thanks!