Despite recent versions of Firefox having colour emoji built in, Debian users will usually see black-and-white emoji characters due to the wrong font being used. ☹️

I wanted to try getting colour emoji working across my whole system, not just Firefox. 🤔

Here is how I did it. ℹ️

Install the emoji font

Before anything else, you'll need to have a colour emoji font installed. The Google Noto font is available in the Debian package repository, so you simply need to run:

apt install fonts-noto-color-emoji

...as root, or via sudo. ⌨️

Uninstall the DejaVu fonts

Note: This isn't strictly required; see the update at the end of this page for details.

The DejaVu fonts have black-and-white versions of most emoji, which cause them to get used in place of the colour ones most of the time. The simplest option is to uninstall them:

apt remove fonts-dejavu-core

Make sure that all related packages are removed:

fonts-dejavu
fonts-dejavu-core
fonts-dejavu-extra

Unfortunately, some (non-font) packages - such as blender or openmw - rely on these being installed, so you'll need to uninstall those as well. ⚠️

Add a configuration file

The following is a modified version of the configuration file described in this Github issue by abouvier. Save this as /etc/fonts/conf.d/56-emoji.conf and all should be well:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
  <alias>
    <family>sans-serif</family>
    <prefer>
      <family>Noto Color Emoji</family>
      <family>Bitstream Vera Sans</family>
    </prefer>
  </alias>
  <alias>
    <family>serif</family>
    <prefer>
      <family>Noto Color Emoji</family>
      <family>Bitstream Vera Serif</family>
    </prefer>
  </alias>
  <alias>
    <family>monospace</family>
    <prefer>
      <family>Noto Color Emoji</family>
      <family>Bitstream Vera Sans Mono</family>
    </prefer>
  </alias>
  <match>
    <test name="family" compare="contains">
      <string>emoji</string>
    </test>
    <edit name="family" mode="prepend" binding="same">
      <string>Noto Color Emoji</string>
    </edit>
  </match>
</fontconfig>

You might need to re-start some program(s) in order for the font configuration changes to take effect, but that's basically it. ☺️

Update (10th June 2019)

Recent versions of Debian implicitly require the DejaVu fonts to be installed when installing a new kernel, or it will fail with a cannot stat '/usr/share/fonts/truetype/dejavu/DejaVuSerif.ttf': No such file or directory error.

To work around this, run dpkg-query -L fonts-dejavu-core to get a list of installed font files:

/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf
/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf
/usr/share/fonts/truetype/dejavu/DejaVuSansMono-Bold.ttf
/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf
/usr/share/fonts/truetype/dejavu/DejaVuSerif-Bold.ttf
/usr/share/fonts/truetype/dejavu/DejaVuSerif.ttf

...and "ignore" them by adding the following to the bottom of /etc/fonts/fonts.conf, just before the </fontconfig> tag:

<selectfont>
    <rejectfont>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf</glob>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf</glob>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSansMono-Bold.ttf</glob>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf</glob>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSerif-Bold.ttf</glob>
        <glob>/usr/share/fonts/truetype/dejavu/DejaVuSerif.ttf</glob>
    </rejectfont>
</selectfont>

This ensures that the fonts-dejavu-core files exist, but don't override fonts-noto-color-emoji with black-and-white versions. 👍