UP2718Q, HDR, Alienware 13-R3, Nvidia 1060

To use the Dell HDR, the computer video card HDMI out must be version 2.0a or higher. The computer video card DP or mDP out port must be version 1.4 or higher.

Dell HDR is a two step process. You have to change both the Windows 10 Display setting and in the monitor OSD (On Screen Display).

When using the Windows desktop or any other non-HDR content, HDR should be Off
* Right click on Windows 10 Display Settings
* Switch HDR and advanced colors to off
* If applicable for your monitor model, change the monitor OSD -Color -Preset Modes to Standard. Not Game HDR or Movie HDR
* If applicable for your monitor model, change the monitor OSD -Display -Smart HDR to Off

When using an HDR Game or Movie, HDR should be On
* Right click on Windows 10 Display Settings
* Switch HDR and advanced colors to On
* If applicable for your monitor model, change the monitor OSD -Color -Preset Modes to Game HDR or Movie HDR
* If applicable for your monitor model, change the monitor OSD -Display -Smart HDR to On, Normal REC 2020, or Vivid DCI-P3

dear Chris, thank you for answer

Windows hdr mode had been switched off all the time.

and hdr mode can be activated whenever connect to HDMI or do

the problem is color differs from those two connecting methods

They are different technologies. They were never meant to be the same. Using DP, you will need to use the Nvidia Control Panel to manually tweak the contrast and color settings. Or use the UP2718Q OSD to manually change the Brightness/Contrast and Color- Preset Mode- Custom Color.