I'll try. First off, have you tried doing the reset smc/pmu and zapping pram on your new macbook Pro 2019 ? Have you tried resetting the monitor and applying any firmware updates to it ?? *as far as I know, you can only do this on windows*. I presume you're using HDMI ....
check to make sure, on the MacBook Pro, under accessibility, that Invert colors is not turned on. Just to let you know, Dell doesn't support Mac at all, and Apple doesn't support Dell stuff. So it's a tossup if it works on Mac or not. At least for monitors. They *mostly* *basically* work. However, there's a problem with the EDID the Dell monitor gives to your Mac, as in, not much. When your Mac talks to the Dell monitor, it has to give back a bunch of info for the Mac to do its job right. There is sorta a fix for it. Google/Yahoo "RGB mode for Dell in OS X". I'm guessing your OS is Mojave. Can't speak for Catalina. That info is available here, although it is for El Capitan
https://www.mathewinkson.com/2013/03/force-rgb-mode-in-mac-os-x-to-fix-the-picture-quality-of-an-external-monitor
I think MST has to be on as well. I'm guessing it's not a problem of the monitor being dim, or a little flat-looking, as having a messed up picture. I think also you have to use an HDMI 2.0 cable, but that's just a guess
anyways, it's a start, right ?
john b