Is my brand new iMac Pro display faulty? There's a dark band beneath the menu bar
No! Your iMac Pro is not faulty! 

I also have an iMac Pro, and below is a screenshot of the same section of the screen. 



My desktop background (i.e. wallpaper) is actually a photo taken by one of this community’s most respected members (so it’s not anything bundled with or generated by macOS) and as you can see the slightly darker shade is also there. This darker shade is actually the menu bar shadow and has been part of Mac OS X since the beginning.



Personally, I wouldn't worry about the shadow. However, if it worries you enough, you have the following built-in options within macOS:


Change your desktop background to an image that camouflages the menu bar shadow (e.g. the bundled Earth and Moon photo)
Go to Apple > System Preferences > General and tick the Use dark menu bar and Dock option
Go to Apple > System Preferences > General and tick the Automatically hide and show the menu bar option
Go to Apple > System Preferences > Accessibility and change your Display Contrast by dragging the slider far enough to hide the menu bar shadow


There are also Terminal commands you can use to disable the shadow effect, however I haven't tested these since probably the Mac OS X El Capitan days, so will test these first before adding them to this answer. 

[UPDATE]

This update confirms I have now tested the Terminal commands that could be used in older versions of macOS to remove the shadow effect from the menu bar and windows. Unfortunately, after testing these in macOS High Sierra 10.13.5, I can confirm they no longer work.

I have also checked the status of older 3rd party apps that used to be able to do this as well, and they do not work in macOS High Sierra either. As a result, the options I provided earlier in my answer appear to be the only workarounds available at this point.