Settings > Display > Navigation Bar > Background Color Options Suddenly Limited

I don't own a Galaxy S8 device, but it seems the recent update has removed or limited the background colours option. From this article it is certainly the case:

Samsung is pushing out a new software update for the Galaxy S8 and
Galaxy S8+ right now, and it looks like the company has decided to
limit the customization options for the navigation bar. Well, at least
when it comes to the choice of color for the navigation bar’s
background, as there is no longer an option to choose a custom color
from an RGB scale. In fact, Samsung has switched up the available
background colors, and the worst of it is that you can no longer use a
fully black navigation bar background.

Again here is the changelog of the update:

You could try other workarounds using apps to extend the range of nav colours as suggested with this app from play store. (NavBar App)