Have you tested this? I don't think it will work because NixOS' version of cairo doesn't support colour glyphs yet, though maybe I'm wrong.

Yes, I have being using it for some months now.
Displaying colored emoji already work in NixOS, the issue was the absence of consistency in the fonts used to displaying emoji. With the option enabled all the emoji are displayed with the emojione font.

Does it work only in the browser or everywhere?

You're right it only work in the browser, other software support only black and white emojii. For the moment I view emojii only in the browser so I didn't checked in other apps.

I don't think this is the right approach. I think we should add fonts.fontconfig.overrides = [ "Noto Color Emoji" ]; that would generate the following config (with priority higher than 73):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <match target="pattern">
    <edit name="family" mode="prepend">
      <string>Noto Color Emoji</string>
    </edit>
  </match>
</fontconfig>

Also, that might not be needed anymore in the latest fontconfig.

Thanks for the comment @yegortimoshenko, that was what I was looking for when I wrote this push request. This PR works but seems dirty. I'll test your way of having emoji on NixOS. However I think there should be a option to enable emoji or at least a clear documentation on how to enable it.

@bricewge I think that was fixed in https://bugs.freedesktop.org/show_bug.cgi?id=94551, i.e. it should automatically discover proper color emoji set w/o falling back to DejaVu.