re: Windows 10 what exactly is the option under Display settings: "HDR and advanced color"?
The quick answer is that HDR (High Dynamic Range) allows for more colors   and better color depth.  Most new UHD Blu-Ray and UHD TV's support the   HDR standards.
That said, good luck trying to figure out the  Windows 10 implementation  of HDR.  I have my PC (with and nVidia GTX  1050) attached to my Vizio  P75 TV.  Enabling HDR on both my TV and  Windows dims the display some,  makes the text seem blurry, but video  seems fine.  If I play and HDR  video from say YouTube, am I really  seeing HDR quality?  Frankly, I  don't know.
HDR comes in two  formats - HDR10 and Dolby Vision.  My TV supports  both.  Blu-Ray  supports HDR10 as the UHD standard with 10 bits per  channel color  support (over 1 billion total colors).  Dolby Vision is  optional on  Blu-Ray with 12 bits per channel color (over 5 billion  colors).
However,  there is very little information on setting up HDR and support   material.  I did a lot of research in the past few weeks on the  subject,  but it's so new there doesn't seem to be any good definitive  test  video.  Watching UHD non-HDR and UHD HDR video sample both look   excellent on my TV.  In fact, one UHD Blu-Ray (Rocky Mountain Express)   is not in HDR actually even though it claims it on the box.
How can I tell?  The only way on my TV is using the included smart  remote and going to the Display Information setting.  From there I can  see the framerate (24p for example), resolution, and whether HDR is  active or not.  But is it HDR10 or Dolby Vision?  There is no way to  know.
Even though movie studios and TV manufacturers are touting the benefits  of HDR, people may or may not even know if they are actually watching  HDR.  That's pretty pathetic in my book.
Take for example the movie Deadpool.  I watched it when I got my new TV  and UHD player.  It looked fantastic.  The video quality, colors, detail  were astounding.  Imagine my surprise weeks later that I didn't even  watch it in HDR because my TV has that option turned off by default.   ???
So back to your original question on Windows 10 HDR mode.  I'm assuming  that it will be used for gaming since DirectX supports 10 bit color.   There is no documentation from Microsoft what the HDR does or even if  it's calibrated properly.
Hopefully soon we'll see more information on HDR.  I really want to see a  calibration disc like Disney's Wow or something so I can be sure I'm  watching HDR video correctly.