When holding a banner in the main hand in creative mode and accessing the survival inventory, the background colors of the player's armor slots darken to black with dark grey outlines for the armor items.
Also, whether or not the banner is being held, when mousing over to the rest of the inventory (above the hotbar), the offhand slot will darken in a similar way.
Thank you for your report!
We're actually already tracking this issue in MC-161847, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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