Ok let me try to explain what I have learned over the last couple of years about HID lights.
All HID lights in cars uses Xenon gas only, as it is the only gas that can illuminate at full, or near full brightness immediately when an arc is struck. If you have seen a non-xenon HID light source, such as the lights in a gymnasium, it takes a long time to reach full intensity. The light emitted from HID Xenon light sources can vary from a bluish tint, like most cars on the road today, to a pure white light, like the Acadia's, or a slightly yellowish tint (new unburned HID light source). The gas in the bulb will change over time, so the colour of the light will change as well. The gas and the mercury have to be "broken-in", for a lack of terms, because when the mercury is injected in the lamp, it is in liquid form, and an initial burn is required to turn the liquid mercury into gas, which will take a while to finish completely. You can see in some Lexus' that the colour between the headlamps of two HID cars can vary, and also in gymnasiums with HID lights that range from yellow to pure white.
The Acadia's do use a Bi-xenon HID light source for both low/high beam. If you look closely at the headlamps of an HID Acadia and a Halogen Acadia, you will see that with the HID Acadia, the two inner projector lamps do not have a true projector lens, as they only function as DRL's, not brights, which is why some owners are complaining of no Flash to pass during the day, while the outer 2 with projector lenses are hi and low beam, using a solenoid to switch between the two functions. That same solenoid is also linked to an adjustment screw, which is what GM uses to aim the headlamps.
A halogen Acadia has 4 true projector lenses, because the inner lamps function as DRLs/Brights, so they have to be aimed properly if used, while the outer two function as low-beam only. Also, I have been lead to believe that some headlamp assemblies have lenses that are tinted blue, like the Enclave while others, like the Acadia, have non-tinted lenses, giving off a pure white light.
I hope that clears things up. I hope to visit my local GMC dealer soon to verify in fact that the Acadias do have a true Bi-Xenon HID light source. Until then, this is all of my own research.
Hope this helps! :thumb: