Most KDE5/Qt5 application now support color emoji fonts. Noto Color Emoji
(noto-coloremoji-fonts) is installed by default but won't take effects. The
reason is that Noto Emoji (noto-emoji-fonts, a monochrome emoji font) is
installed by default, too and has a higher priority.

Test A:

1. Make sure you installed both noto-coloremoji-fonts and noto-emoji-fonts.
2. Start Chromium or Falkon. (Firefox doesn't support Noto Color Emoji and has
its own built in EmojiOne Mozilla font)
3. Visit https://getemoji.com/ to check emojis.

Result: most emojis are monochrome (using noto-emoji-fonts)

Test B:

1. Make sure you installed noto-coloremoji-fonts and uninstalled
noto-emoji-fonts.
2. Start Chromium or Falkon. (Firefox doesn't support Noto Color Emoji and has
its own built in EmojiOne Mozilla font)
3. Visit https://getemoji.com/ to check emojis.

Result: most emojis are colorful (using noto-coloremoji-fonts)

DejaVu fonts also provide many monochrome symbols and have a higher priority
than emoji fonts.

Several years ago, when color fonts are not widely supported, having a
monochrome emoji font is safe. But now, most browsers and messaging
applications support color emoji fonts. Maybe it is time to drop
noto-emoji-fonts and dejavu-fonts from default system installation.