I've owned an LG 4K HDR TV for a few weeks now and initially I found getting the HDR picture to look its best quite baffling. The solution is surprisingly simple though. After lots of fiddling with all manner of controls I've discovered that avoid the picture being too dark in HDR UHD disks or HDR Xbox One S games is merely a matter of adjust the TV brightless level. I wrote the following response on a forum but I thought it was worth sharing here.
Update: Since I first posted this, LG has released a firmware update to add a low-latency HDR mode for gaming. This new mode appears alongside the other three HDR modes.
In HDR mode use "Normal" gamut (unless your TV is an LG OLED). In HDR mode this is actually "wide" (yes I know it's bonkers). But I suppose LG's reasoning is that this is the "normal" gamut for HDR, which is a wide gamut.
HDR Standard (dark mode) is fine. But it's taken me a few weeks of owning my 49UH805V to solve issues of HDR calibration.
First in SDR/HDR stick with LG's default. They know their TV better than the internet. So start with the baseline in SDR of the following:
White Balance: Warm2
Led Local Dimming: Whatever the default is
All other processing off, though truemotion user: 0/10 isn't a bad setting.
Then just adjust the backlight depending on the room. Lower than 30 in a very dark room, much brighter in a bright room.
Don't copy someone's detailed white balance settings. There really is no point.
For HDR things are a bit more complicated. HDR Standard can seem a bit dark. Over the last few weeks I've over complicated these settings but now I reckon I've got a good handle on things, and I've seen other comments that make me believe I've got this right. It's really simple.
First make sure your Xbox One S is set to standard not RGB. There really is no point setting it to RGB, especially if you watch media as well as play games. So set the Xbox to standard and the TV black level to low.
For HDR start with the TV's defaults. So do a reset to remove any of the settings you've added. We'll start with HDR Standard (dark). Remove any of the picture processing like noise, anything dynamic and the like. But keep the default LED Local Dimming level, perhaps Truemotion to User 0/10. Also set sharpness 10 to 15. Don't set it at zero, that is not zero sharpness on LG TVs but minus sharpening ie blur.
Now initially on any HDR input this can seem a bit dark. It seems that different sources crush the black level. One solution is adding Dynamic Contrast, the HDR Bright mode does just that. And Dynamic Contrast is okay to use in HDR modes, it's not the disaster it is in SDR. However I reckon that you should only use HDR Bright mode in a really bright sunny room when my following advice doesn't work. So in HDR Standard don't use Dynamic Contrast, just try the following.
So what do we do to make HDR Standard look good? It's really easy. Just adjust brightness to avoid the darks crushing. Whether playing Xbox One S stuff, UHD Blu-rays or watching The Grand Tour on Amazon. Setting brightness on the TV between 52 and 54 seems to do the trick. It's amazing how much better HDR Standard is if you do this. The number you need does seem to vary between sources. For example The Revenant UHD disk crushes black quite a bit but looked fabulous in HDR Standard with brightness at 54. But other sources 52 was fine.
So follow the same for playing Forza Horizon 3. Just use HDR Standard (as long as your room isn't too bright) but just adjust the brightness on the TV up a few notches from 50. You'll be amazed how much difference it makes.
What about the HDR settings in Forza Horizon 3? Start with them at default. Dont' lower the in game "brightness". Raise the in-game "HDR brightness" setting, but ignore the advice about making the logo disappear. Just raise it until the sunlit clouds blow out a bit, then back it off so the clouds don't blow out (somewhere between 10 to 20 notches from zero). Also do this setting with the game in an external camera, the windscreens in cockpit view lower the brightness as they are tinted, so a true reading of whether the sunlit clouds are blown out is from a third-person camera.
Hope this helps. Forza Horizon 3 in HDR looks fabulous on my TV in HDR Standard with the TV brightness raised a few notches above the default of 50. And you don't need to use wide gamut, as HDR is already wide.
I suspect this advice will work just as well for users of HDR televisions other than LG. Hope this helps.