Why HDR looks washed out, and how to fix it

Start with what looks washed out. If it's the desktop and normal apps, that's SDR content shown in HDR mode. Windows fits SDR content into the HDR signal and sets its brightness with the SDR content brightness slider in Settings, System, Display. With the slider low, white looks like a dim gray. Move it until a white page looks the way it did with HDR off.

If HDR movies and games look flat too, look at the screen. HDR needs contrast. An LCD without local dimming has one backlight level for the whole picture, so it has to stay bright for the highlights and lifts every dark part with it. That's why a monitor with a loose black level, like DisplayHDR 400 with its 0.4 nit limit, can look flatter in HDR than in SDR.

Then check the monitor and the cable. Some monitors switch to a separate HDR picture mode with its own brightness and color settings, and eco or power saving modes can cap brightness. A cable or port that can't carry 10-bit color at your resolution and refresh rate can make the PC drop to a lower bit depth or to 4:2:2 color, which can add banding and soften text.

Run the compare test with SDR white on the left and 1,000 nits on the right. If the right patch is clearly brighter, HDR works and the problem is how SDR content is mapped. If both look the same, the screen isn't showing HDR, or SDR white already sits near its peak.