Next month's update will fix some of this, as it'll let you specify the level of HDR brightness you want for your specific screen. I've scoured the web to find out that this is what is happening, I cannot recall where I've read it but I've seen references to this in a few places in the past 6 months. That affects brightness AND colors, which is why it all looks faded, dim and weird. Windows 10 right now, on the 1709 version, limits the desktop to 100nits when you switch on HDR on a display that supports it. This is the only correct answer in this thread.