Why would a lock screen in Windows 10 grow less vivid after being set and forced to not be changed in group policy?

I setup a *.png file from a scan that is in a bright orange color.  When I set it as my default lock screen in Windows 10 and set group policy to not allow it be changed, it changes it, saves and locks the image to not be changed by the user.  For some reason when the user logs off the image is darker and with a less vivid color of orange even though it appears bright under Settings -- Personalization -- Lock Screen.  Any ideas why this would happen and how to set it to be brighter?
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JohnBusiness Consultant (Owner)Commented:
For the screen (an image) to darken has become normal in Windows 10 (numerous machines). I have been looking for a setting on this but have not yet found one.
JohnBusiness Consultant (Owner)Commented:
Go here and adjust Dimmed Brightness higher than 50 %

Shaun VermaakTechnical SpecialistCommented:
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JohnBusiness Consultant (Owner)Commented:
I tried the setting myself and it improves things. However Windows itself dims the display at sign in / log off and that feature apparently cannot be disabled.

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JohnBusiness Consultant (Owner)Commented:
Thanks. The setting I pointed to is helping the dimness issue.
Qlemo"Batchelor", Developer and EE Topic AdvisorCommented:
IIRC 1709 introduced an adaption for color profiles with the intention to make photos more realistic in display, which led to a significant darker layout with some VGA and monitor combinations. Adjusting the color profile of the monitor could help.
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