@Mark_VG it turns out that Philips/Signify don’t actually support color, or color-temperature states or actions on light groups (rooms/zones/all). For groups the only actual channels are the switch and brightness channels. You can actually see this in the Hue App. So it is in fact ‘correct’ that groups would display the state of such channels as ‘null’. And therefore I think I shall to remove the color and color temperature channels from groups in OH as well.
Just for info: it looks like the brightness of a group is the arithmetic average over the lights within it; and the switch state of a group is the logical OR over the lights within it. And I guess the complexity of ‘averaging’ the color or color temperature over several lights might explain why Philips / Signify does not do this. Although in the App if lights in the group do have different colors, the App displays a kind of amalgam of the two..
As you mentioned earlier, the only way to set a color or color temperature for a group, is to activate a scene on that group. However the scene does not set a color or color temperature on the group; it actually sets color or color temperature for the individual lights only.