Yeah, "warm display colors" is not at all obvious! Who picked this name.
A google search "Z5 ii astrophotography red display" finds all kinds of "red" but not the display setting. It's probably buried in some of the 15-20 minute video links, but which ones?
Searching the online manual, "astro" has no hits. but "astronomical" is used on the warm display colors page, so it finds it. It has to be an exact match though.
I knew about it from watching long "how to set up the Z6 iii" videos, going through all the menus, and devoting 15 seconds to this setting. I think a few DPR threads covered it when the Expeed 7 cameras were new too.
Oh, this works:
"nikon z5 ii red display for night" -- the AI summary is correct (it's often quite wrong for these technical questions.)
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For very dark conditions, My Z6 iii has these settings (Z5 ii links here):
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d12: warm display colors. I prefer the Mode 1,"everything is red" choice. I have the brightness set to +1 from the last time I used it.
https://onlinemanual.nikonimglib.com/z5II/en/csmd_warm_display_colors_241.html
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Sometimes, I turn down the screen brightness to near minimum, instead of setting d12:
https://onlinemanual.nikonimglib.com/z5II/en/sum_monitor_brightness_314.html
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d11: Starlight view. This is actually too bright for night shooting in the city. It's fantastic for an astro scene. It's way better with a tripod, since the display is built up over 5-6 seconds, and handheld will blur the view.
I focused on a person and telescope using Starlight view, when I could barely see them at all.
https://onlinemanual.nikonimglib.com/z5II/en/csmd_starlight_view_photo_lv_240.html