Is there a distribution or flavor of *nix that stands above the rest in terms of available apps in the repository, smooth workflow, colour management, etc?
There isn't one, overall. I don't think anything focuses on this specifically. Any distro can work just fine, and I suggest picking one that you like in general and the photography apps will follow.
That said, there are two considerations that are probably worth... considering.
First, unless you plan to find and compile various software yourself, you want a distribution with a wide array of existing packages. Pretty much any distro with a desktop focus will have Gimp, but they may not all have Darktable or Rawtherapee or other raw converters. That means pick one of Fedora, Debian, Ubuntu, or openSUSE — or a smaller distro based right on one of them and which can use the same packages directly.
Both Ubuntu and Fedora offer variants with preselected software for multimedia content creation — Ubuntu Studio or the Fedora Design Spin. These may provide a quick starting point because of their preselected software, but once you're up and running don't, they're really not that different from starting with the "generic" software and then adding the photography apps from the general repositories (which is very easy to do with any modern distro).
Second, following the same, a distribution that moves fast and has frequent releases is probably better, because a lot of this software is still in early development and improves quickly. Really, each of the above offers a way to get this, although it is more of an explicit focus for Fedora than the others.
Disclaimer: I work on Fedora. And I will give a little bit of biased advice expressed after my general comments, since you mention color management. The GNOME desktop environment has the easiest color management, and since Red Hat employs many GNOME developers, I think it's fair to say that Fedora is the best GNOME distribution. (You don't have to take my word for it... look around.) You can use various color management systems in any distro, but
gnome-color-manager make it easy. And particularly, the author of this system, Richard Hughes, works on Fedora. (Richard also makes the open source colorimeter ColorHug, which will work out-of-the-way with Gnome.) So, while these improvements eventually always hit other distributions, I think it's fair to say that we lead the way here.