I don't use Spotlight or Lock Screen. I just use the default sign-in screen that is blue by default on Windows 10 and is grey (or black?) by default on Windows 11. How do I change its color? It cannot be changed in any Theme or Display or Personalization settings. I think there is a registry entry for it, but I don't know what it is. I'd prefer to have it grey or black like in Windows 11.
If you really are a real person and not another spammer, as is often the case lately, then yes, you can also do it in the registry, but it is very troublesome, because it is there in several places and also encoded. It is much easier to open Settings => Personalization => Colors and go to Choose your accent color. There you can choose a color from the table or make your choice by clicking Custom color below the table, and then choose any color or write the desired color code manually and it will be saved wherever you need it automatically. This has been the case for many years, although at the very beginning, when Windows 10 was released, this option (custom color) was not available. Then was only predefined colors table there.
Thanks! Those don't adjust the Sign In screen color. If I knew the HEX value of the (default blue) Sign In screen color, then I could search registry for that value to find out which registry entries belong to the Sign In screen.
Windows 10 default blue color is: HEX: #00adef or RGB(0, 173, 239) or RGBA(0, 173, 239, 1.0) or but Windows doesn't uses any of them, Windows uses in registry this 0xff0078d7, where the 0x is prefix and ff is the opacity (it can be from 00 to ff, 00 is completely transparent, and ff is fully opaque.) (also known as alpha) value is ignored and has no bearing on the color.
There's a less elegant workaround you can use to set a sign-in background color independently from the accent: Re-enable lock screen (undo policy/apply), so you can manually select a picture background for it (ex., a small background png tile of your intended color) via settings gui. Then, you can disable the lock screen again but enable the toggle to "show lockscreen background picture on the sign in screen". You might wish to use a slightly brighter color, as the sign-in screen will add a subtle dark overlay over your background picture.