Will this conflict with my existing User Name Color install?
No. It uses the same add-on ID, so XenForo treats it as an upgrade rather than a second add-on. Your styles, member colours, options and permissions all carry over.
I installed it and my names look the same as before.
The CSS lives in a generated template rather than a shipped file. Upgrading rebuilds it automatically, but if you've moved servers or restored a database since, press Rebuild CSS on the style list, then hard-refresh. If you're behind a CDN, purge it — the stylesheet URL only changes when the template does.
My user group styling stopped showing on members who have a name style.
That's intentional. Turn off Name styles replace user group styling in the options to get the old layering behaviour back.
Can I go back to the version I was on?
Yes. It's the same add-on ID, so reinstalling the older release over this works — but its version ID is lower, so you'll need to do it manually rather than via an update prompt.
Does this work on XenForo 2.1 and 2.2?
It's built against 2.1.0+ and the fixes are 2.3-focused, since that's where the breakage showed. It should be fine on 2.1 and 2.2, but production testing has been on 2.3.
Why is the add-on ID still ThemeHouse/UserNameColor?
So it upgrades in place instead of installing alongside your existing copy and fighting it. Changing the ID would mean every existing user losing their styles and member colours on migration.