Certain applications (firefox, chrome, spotify, ...) already support displaying colored emoji. GTK+ (cairo) doesn't, and I'm not sure about Qt.
This fontconfig file adds Noto Color Emoji (noto-sans-ttf) to every query because it has only emoji glyphs and nothing else. It also adds EmojiOne color font to Firefox, the only program (that I know of that supports SVGinOT). EmojiOne does not have a package, so it needs to be packaged (just the ttf, without its provided fontconfig file).